How did the Virgin Birth of Jesus prove to be a “sign” as prophesied in Is 7:14?

How did the Virgin Birth of Jesus prove to be a ”sign” as prophesied in Is 7:14?

The virgin birth of Jesus is a sign to believers that Jesus’ birth was willed by God in a miraculous manner and is mark of Jesus’ divinity and humanity, at the same time, without the natural aid of any man!

The doctrine of the virgin birth teaches that Jesus Christ was born of a virgin. That is, when Mary conceived Jesus, she had never had sexual intercourse. Jesus’ birth, therefore, was truly miraculous. The virgin birth of Jesus is a crucially important doctrine and one that the Bible plainly teaches in Matthew 1:23 and Luke 1:27, 34.

Let’s look at how Scripture describes the virgin birth. The angel Gabriel visits the Virgin Mary to bring her the news that she would be the mother of the Messiah. Mary asks, “How will this be, since I am a virgin?” (Luke 1:34, ESV). Gabriel’s reply indicates the miraculous nature of the conception: “The Holy Spirit will come upon you, and the power of the Most High will overshadow you. So the holy one to be born will be called the Son of God” (Luke 1:35). The angel points not to any human act but to the Holy Spirit and the power of God as the agency of Jesus’ birth. Jesus would properly be called the Son of God.

Gabriel later repeats the news to Joseph, betrothed to be married to Mary: “What is conceived in her is from the Holy Spirit” (Matthew 1:20). Joseph needed this information because, “before they came together, [Mary] was found to be with child through the Holy Spirit” (Matthew 1:18). Accepting God’s word on the matter, Joseph proceeded to take Mary as his wife, but she remained a virgin until after Jesus was born: “He did not consummate their marriage until she gave birth to a son” (Matthew 1:25).

The virgin birth of Jesus Christ was predicted in the Old Testament: “The Lord himself will give you a sign: The virgin will conceive and give birth to a son, and will call him Immanuel” (Isaiah 7:14, quoted in Matthew 1:22). There is also a possible allusion to the virgin birth in Genesis 3:15, which says that the “seed” of “the woman” would destroy the serpent.

The virgin birth of Jesus is an example of God’s gracious work on our behalf. God took the initiative—Mary was not looking to become pregnant—it was all God’s idea. Joseph had no role in the conception—his body was not involved—so the power had to come from God. In a similar way, our salvation is based solely on God’s initiative and God’s power—we did not seek God, but He sought us; and we did nothing to earn our salvation, but we rely on God’s power.

Unsurprisingly, Jesus’ enemies among His contemporaries denied His virgin birth. They went so far as to publicly accuse Jesus of being a Samaritan, i.e., a person of mixed race (John 8:48). Those today who would deny the virgin birth contradict the clear teaching of Scripture, call into question other miracles recorded in the Bible, and open the door to a denial of Christ’s full deity or His full humanity. - Why is the virgin birth so important?

For some Mary is the Great Sign (Signum Magnum) that appeared in the heaven in the Book of Revelation.

12 And there appeared a great wonder in heaven; a woman clothed with the sun, and the moon under her feet, and upon her head a crown of twelve stars:

2 And she being with child cried, travailing in birth, and pained to be delivered.

3 And there appeared another wonder in heaven; and behold a great red dragon, having seven heads and ten horns, and seven crowns upon his heads.

4 And his tail drew the third part of the stars of heaven, and did cast them to the earth: and the dragon stood before the woman which was ready to be delivered, for to devour her child as soon as it was born.

5 And she brought forth a man child, who was to rule all nations with a rod of iron: and her child was caught up unto God, and to his throne.

6 And the woman fled into the wilderness, where she hath a place prepared of God, that they should feed her there a thousand two hundred and threescore days.

7 And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels,

8 And prevailed not; neither was their place found any more in heaven.

9 And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him.

10 And I heard a loud voice saying in heaven, Now is come salvation, and strength, and the kingdom of our God, and the power of his Christ: for the accuser of our brethren is cast down, which accused them before our God day and night.

11 And they overcame him by the blood of the Lamb, and by the word of their testimony; and they loved not their lives unto the death.

12 Therefore rejoice, ye heavens, and ye that dwell in them. Woe to the inhabiters of the earth and of the sea! for the devil is come down unto you, having great wrath, because he knoweth that he hath but a short time.

13 And when the dragon saw that he was cast unto the earth, he persecuted the woman which brought forth the man child.

14 And to the woman were given two wings of a great eagle, that she might fly into the wilderness, into her place, where she is nourished for a time, and times, and half a time, from the face of the serpent.

15 And the serpent cast out of his mouth water as a flood after the woman, that he might cause her to be carried away of the flood.

16 And the earth helped the woman, and the earth opened her mouth, and swallowed up the flood which the dragon cast out of his mouth.

17 And the dragon was wroth with the woman, and went to make war with the remnant of her seed, which keep the commandments of God, and have the testimony of Jesus Christ. - Revelation 12

The question also begs another question as to why did the Blessed Virgin Mary marry?

The Virgin Mary married St. Joseph comes directly from Scripture. When Joseph found out Mary was with child, decided to put Mary away privately in order to spare Mary the shame of having a child out of wedlock. In taking Mary as his wife, the Holy Family was looked upon as any other Jewish family living in Palestine at that moment.

Origen gives us another reason why Mary and Joseph married one another. Mary’s virgin birth of Jesus was not noticed by Satan due to her marriage with St. Joseph.

To begin in the East, in his Homilies on the Gospel of Luke, Origen considers the perpetual virginity of Mary already a settled point of doctrine, fully consistent with the biblical testimony (see Homily 7.4). He also points out that the reason Mary was frightened by the greeting of the angel, “Hail, full of grace,” is that she, as a good student of the Law, knew that this form of address was unprecedented in Scripture (Homily 6.7). In other words, Origen is pointing out that Mary’s participation in grace is unique, and this conviction continues in both Eastern and Western Christianity, a grace of sinlessness in the East, and Immaculate Conception in the West. For Origen, however, the uniqueness of Mary is not isolated from her marriage to Joseph. His comments on her uniqueness come immediately after a discussion of the divine dispensation which committed the Incarnation to a woman who was already betrothed. In Origen’s reading, then, the marriage of Mary and Joseph is not accidental to the divine plan, but part of it, and so itself becomes theologically significant. It is not Teresa of Ávila, but Origen, who first makes the mystery of St. Joseph and his marriage to Mary an intrinsic part of the mystery of the Incarnation Origen reports:

I found an elegant statement in the letter of a martyr - I mean Ignatius, the second bishop of Antioch after Peter. During a persecution, he fought against wild animals at Rome. He stated, "Mary’s virginity escaped the notice of the ruler of this age." It escaped his notice because of Joseph, and because of their wedding.

Origen continues to ponder the mystery of St. Joseph, pointing out that it is because he is the husband of Mary that the devil does not suspect that the Savior “had taken on a body.” Origen connects the mystery of St. Joseph with Paul’s reference in 1 Cor 2:6-8, where, as Origen reports, Paul comments that:

We speak wisdom among the perfect, but not the wisdom of this age or the wisdom of the rulers of this age. They are being destroyed. We speak God’s wisdom, hidden in a mystery. None of the rulers of this age knows it. If they had known it, they would never have crucified the Lord of glory.

The marriage of Mary and Joseph is thus an intrinsic part of God’s Wisdom, an intrinsic part of the logic of the Incarnation, which is the logic of God’s philanthropia, a logic of foolishness, invisible to the ruling powers because it is, to them, foolishness and not wisdom. Origen invites us to contemplate the marriage of Joseph and Mary as an outcropping, one might say, of this foolishness. - The Sex Life of Joseph and Mary

It might be worth looking at a bit of historical context here. The beginning of the chapter reads:

And it was told the house of David, saying, Syria is confederate with Ephraim. And his heart was moved, and the heart of his people, as the trees of the wood are moved with the wind (Isaiah 7:2).

As you likely already know, the tribes of Israel split upon King Solomon's death--Judah and Benjamin forming one nation, with Jerusalem as its capital, and the remaining ten tribes forming another, which is sometimes referred to in the scriptures as "Ephraim." Here, Ahaz (the king of Judah) is being told that the rival kingdom next door has allied itself with one of Judah's biggest bullies, Syria. That's a frightening thought!

Isaiah is told by the Lord to go visit Ahaz and give him comfort. "I know you fear this confederacy," Isaiah tells him, "but nothing is going to come of it. In fact, Ephraim isn't even going to be a country in sixty years or so!" We don't see Ahaz's response to this remark, but one might imagine he'd be skeptical. Regardless, the Lord sees fit to reassure him.

¹¹ Ask thee a sign of the Lord thy God; ask it either in the depth, or in the height above. ¹² But Ahaz said, I will not ask, neither will I tempt the Lord. ¹³ And he said, Hear ye now, O house of David; Is it a small thing for you to weary men, but will ye weary my God also? ¹⁴ Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel (Isaiah 7:11-14).

This exchange seems to suggest that this sign is for Ahaz. It is obviously recorded for others to read, and scripture frequently is of such a nature that it can have many different meanings in different time periods or contexts to match the different needs of those who read it (for example, I wonder if this scripture was of comfort or help to Joseph, Mary, or any of the others who knew of the nature of Christ's birth as it happened). Nonetheless, the principle audience of this conversation is Ahaz. If that is the case, then it begs the question--how is any aspect of Christ's birth a sign to Ahaz? Barring a Methuselaic lifespan, he will not see Christ's birth himself. The sign that Ahaz received from Isaiah was not one in the sense we often imagine (angelic visitations, fire from heaven, etc.), but the testimony of Isaiah and subsequent witness of the Holy Ghost of the reality of the Messiah, the one whom the Law of Moses had pointed Israel toward for generations. With sufficient faith in Him, Isaiah implies, no physical danger--no mortal enemy--is truly of the kind of pressing concern Ahaz seemed to be feeling. Because of Him, all that is wrong will be made right, and all that suffer will be exalted. To Ahaz, that sign was more important, impactful, and comforting than any other reassurance Isaiah could have given.

OP: How did the Virgin Birth of Jesus prove to be a “sign” as prophesied in Is 7:14? and how could a layman who had seen Mary living with her duly-wedded husband, know the difference?

The sign itself is the virgin conception and birth, but it unfolds over a sequence of events and people from even before Mary.

Isaiah prophesied some 700 years earlier (first prophet). Daniel prophesied some 450 years earlier. Malachi (last prophet until forerunner) had prophesied some 400 years earlier.

Thus, Mary, Joseph, and many, many others knew the time was coming for fulfillment. Indeed, scripture makes this expectation clear (see Luke 2:38, John 11:50)

Fulfillment begins with the prophesied forerunner by a woman barren (Lk 1:13-17) Mary is espoused and found with child of the Holy Spirit (Mt 1:18)* Joseph is informed (Mt 1:20)
Elizabeth, married to a Levitical priest, finds out (Lk 1:40-45)

From the fulfillments (both miracle conceptions like Abraham/Sarah barren and as a virgin), it continues with the birth of Christ.

Shepherds find out (Lk 2:8-9)* The shepherds spread the word (Lk 2:17-18) The wise men are informed (Mt 2:1-2) Simeon prophesies (Lk 2:25-35)* Anna prophesies to all looking for Him (Lk 2:38) Herod and all Jerusalem find out (Mt 2:3) The chief priests and scribes find out (Mt 2:4) Those in Bethlehem environs find out (Mt 2:16)

To amplify what Simeon and Anna said.

And she coming in that instant gave thanks likewise unto the Lord, and spake of him to all them that looked for redemption in Jerusalem. Luke 2:38

The point is the expected sign of a virgin conception and birth was very well and widely known. Its fulfillment was also well and widely known.

38. coming in--"presenting herself." She had been there already but now is found "standing by," as Simeon's testimony to the blessed Babe died away, ready to take it up "in turn" (as the word rendered "likewise" here means). to all them, &c.--the sense is, "to all them in Jerusalem that were looking for redemption"--saying in effect, In that Babe are wrapt up all your expectations. If this was at the hour of prayer, when numbers flocked to the temple, it would account for her having such an audience as the words imply [ALFORD]. JFB here

The excitement of the fulfillment subsided for about 30 years. Some had looked for the coming of a warrior king, not what turned out to be the fulfillment of the suffering servant.

About Jesus' age 30, then more is said. They discover the fulfillment of the forerunner who confirms what had happened 30 years prior. It was done in the open.

John the Baptist finds out and preaches widely (Mt 3) Jesus calls His disciples who spread the word The gospels are written to spread the word Paul is raised up to spread the word The remainder of the New Testament is written (and assembled I believe)

After Jesus' ascension, then more is said.

The early church elders spread the word And on it goes to today.

So, how would someone know that the virgin conception and the birth was fulfilled?

None of it was hidden from sight. Isaiah's prophesy of a sign was there. Daniel's time frame was there. Malachi's prophesied forerunner was there. They knew and expected God to fulfill His words.

To be sure, some believed, while some did not. Some misunderstood Isaiah's sign. Some believe Daniel's time frame continues. Some believe Malachi's sign continues. But, obviously, many of those who were present at the time, knew, believed, understood, and left us the truthful record.

It was for our benefit

Many of the Old Testament scriptures regarding the Messiah were actually for the benefit of Christians after the resurrection, as per 1 Peter 1:10-12:

1 Peter 1:10-12 CSB Concerning this salvation, the prophets, who prophesied about the grace that would come to you, searched and carefully investigated. [11] They inquired into what time or what circumstances the Spirit of Christ within them was indicating when he testified in advance to the sufferings of Christ and the glories that would follow. [12] It was revealed to them that they were not serving themselves but you. These things have now been announced to you through those who preached the gospel to you by the Holy Spirit sent from heaven-angels long to catch a glimpse of these things.

Also, Jesus's family

Jesus's family members (James, Jude) seem to have been important church members, so it being a sign to them isn't trivial.

Mary herself having scriptural support for her experience can't have hurt when/if she questioned things.

Mary's virginity and conception of Jesus by the Holy Spirit are not circumstances that were published abroad until after the death, burial, and resurrection of the Lord Jesus. Indeed, Joseph's entire encounter with the angel of the Lord appears geared toward keeping the matter a secret. The angel reminds Joseph of Isaiah's prophesy and yet, rather than a command to publish it abroad to Israel, Joseph is encouraged to move forward as planned and take Mary as his wife:

Then Joseph her husband, being a just man, and not willing to make her a publick example, was minded to put her away privily. But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost. - Matthew 1:19-20

Thus the virgin birth cannot have been given as a sign to the immediate generation of people surrounding Jesus for, as OP has astutely noted, it was kept secret. This sign was only published abroad after God's judgement of sin was finalized on the Cross and the Virgin birth of Jesus is a sign to the entire world, beginning in Judah, of God's impending judgement:

And Simeon blessed them, and said unto Mary his mother, Behold, this child is set for the fall and rising again of many in Israel; and for a sign which shall be spoken against; (Yea, a sword shall pierce through thy own soul also,) that the thoughts of many hearts may be revealed. - Luke 2:34-35 

The overarching context of Isaiah 7 and 8 appears to be God's judgement of His unbelieving people through Assyria and the "sign" of Immanuel is the tipping point between deliverance and judgement.

Ahaz (and all Judah) were afraid "because Syria, Ephraim, and the son of Remaliah, have taken evil counsel against thee (7:5)", but the Lord has said "It shall not stand, neither shall it come to pass (7:7)" and also "If ye will not believe, surely ye shall not be established (7:9)".

Ahaz, as representative of the House of David, is instructed to ask for a sign and Ahaz refuses to do so. God has mercifully offered to grant a sign to strengthen the belief of Ahaz (and Judah) and, thus, establish them as per verse 9. Ahaz refusal to ask is essentially Ahaz refusing to lift his eyes up from immediate circumstance and to trust in the Lord: He did not want his belief strengthened to endure the immediate challenges to come. How very like us to desire immediate rescue over strength to endure.

Therefore the Lord gave him a sign and this was a sign of an even greater judgement: Isaiah's wife was to conceive and bear a son and before this son was even old enough to "know to refuse the evil, and choose the good (7:16)" the King of Assyria was to come and conquer Israel and Judah. This is referred to in verse 16 as "the land that thou abhorrest" because, in refusing to look to God for assurance, God's people were bringing judgement upon the land.

Thus the sign of Immanuel is "God with us" but it is a sign of judgement. Jesus, born of the virgin Mary, is pointed to as the greater fulfillment of this sign for in His body has all of sin in the flesh been judged (Romans 8:3) and the Holy Spirit has been sent out to convince the world of sin, and righteousness, and judgement (John 16:8-11).

This greater sign is given to the whole world and, like Ahaz, we may either ask to receive it or refuse to ask. Receiving the sign of the Son of Man provides the deliverance from judgement that God has promised (It shall not stand, neither shall it come to pass) and refusing this sign brings about greater judgement (If ye will not believe, surely ye shall not be established).

For the Father judgeth no man, but hath committed all judgment unto the Son: That all men should honour the Son, even as they honour the Father. He that honoureth not the Son honoureth not the Father which hath sent him. Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life. - John 5:22-24

An evil and adulterous generation seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas: For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth. The men of Nineveh shall rise in judgment with this generation, and shall condemn it: because they repented at the preaching of Jonas; and, behold, a greater than Jonas is here. The queen of the south shall rise up in the judgment with this generation, and shall condemn it: for she came from the uttermost parts of the earth to hear the wisdom of Solomon; and, behold, a greater than Solomon is here. - Matthew 12:39-42