How it is possible to be one God in three persons (Jesus, Holy Ghost, and God the Father)?

Perhaps the key to understanding the Trinity is to consider that God has the ability to extend Himself, and the two extensions of God are Jesus and the Holy Spirit. Think of God as a tree and it having two branches, those branches are Jesus and the Holy Spirit. Three Persons, but still One God.

Years ago this concept was put to me like this, being one person I have many roles in this life:
1) When I was created in my mother's womb, I became a son
2) Years later, I got married, so then I became a husband
3) And soon thereafter, my wife and I became parents, me being the father.

So with those 3 roles (and I have many more), I have different responsibilities, but I am still one person. I know this is way oversimplification of a very complex issue, it did help me understand a little the concept of God being 3 in 1. And, I also came to the realization we humans are finite beings here on earth. Our God is Infinite. Without finite minds we cannot possibly fully grasp the infinite possibilities of an infinite God.

According to trinitarians, God is One because there is only one uncreated substance in all of reality. That one uncreated substance is also three persons.

There are two crucial points you have to understand to make sense of this seemingly contradictory language about God.

1. The term "person" does not mean "being."

2. "God is One" refers to His substance, not to the fact that He is tri-person.

Also related: How does Jesus' pre-incarnational existence relate to oneness pentecostalist godhead doctrine?

Consider the following analogy. The Greeks established the parallel line postulate, and Euclid wrote of it. He and succeeding mathematicians were perplexed why they could not use their other postulates and axioms to prove it. Then Lobachevsky and other mathematicians discoverd non-Euclidean geometries where parallel lines DO meet. The fact of whether parallel lines meet or not is a basic axiom of your mathematics and your world. It is a given, it does not have a cause. There is no why to be answered.

The Old Testament strongly emphasizes the unity of God. There is one God, not many.

The New Testament describes the different roles of a Father, a Son and the Holy Spirit. It descibes their unity. Jesus said in John's gospel, "I and the Father are One." It describes their distinctness, such as when Jesus is asked when the Kingdom of Israel would be restored and other prophesied events would occur and in Matthew 24 says:

“Now concerning that day and hour no one knows—neither the angels in heaven, nor the Son —except the Father only."

There are many statements in the New Testament that show the different roles of the members of the Trinity. They are described, but not explained. They are revealed as being. That just is the way God is. Since God is eternal and unchanging, God has always been this way so there is no cause for Him being this way. He just is. Like the parallel line postulate, we live in a universe whose axioms are consistent with the nature of God as He is. God's triune nature is the bedrock on which the universe was constructed.

Why do the Old and New Testament seemingly disagree? If two things are very close to each other and you are far from them, they look like one thing. The New Testament gives us a more up close view of God, allowing us to distinguish features that could not be seen with the earlier information we were given. However, that distinction between the trinity was present in the Old Testament, just not in focus. When Jesus met the two men walking to Emmaus in Luke 24:

25 He said to them, “How unwise and slow you are to believe in your hearts all that the prophets have spoken! 26 Didn’t the Messiah have to suffer these things and enter into His glory?” 27 Then beginning with Moses and all the Prophets, He interpreted for them the things concerning Himself in all the Scriptures.

If the Son of God tells us that it was all spelled out in the Old Testament, but we were too blind to see it, then it must be there, so we should look closer.

"The Father is God, the Son is God, the Holy Spirit is God, they are not three gods but one God' says the Athanasian Creed

The Scripture explains how the Father, the Son and the Holy Spirit are seen as the "one God".

1. One God, one Lord (1 Corinthians 8:6)

In this passage, the Father is said to be the one God (eis theos) whilst Jesus Christ is the one Lord (eis kyrios). The Greek phrase eis theos and eis kyrios are coined by Paul using the Shema of Deuteronomy 6:4 where it says that God (theos) is that Lord (kurios) who is one (eis).

Here, we have seen that the Father and Jesus Christ are both equally having divine titles!

2. Divine Nature (Galatians 4:8).

In Galatians 4:8, Paul says that those gods are "by nature not gods".

In 1 Corinthians 8, the two persons (the Father and Jesus) are both having divine titles. In that text, Paul contrasted them against the false gods. Both the Father and Jesus have the same nature (nature = essential attributes) of being above the all things, being the source of them.

3. One Adam, One flesh (Genesis 1-2, 5:2).

In Genesis 1-2, it says that the female came from the ribs of the male and that God breathed the spirit of life into the male alone. Therefore, the female derived her being (both body and spirit) from the male. In that sense, they are really of same being and of same nature. So the male says that the female is flesh of his flesh and bones of his bones. Also, in their union they are called one flesh. In Genesis 5:2, God called them "Adam" (not Adams). Thus, a singular for two persons signifying their nature.

4. Three persons, One Name (Matthew 28:19)

In this text, the Trinity is of same name (Adonai/YHWH) which signifies their one eternal essence or nature (Exodus 3:14 LXX).

Note

Philippians 2:10-11 calls Jesus Lord (Greek: kyrios) in the sense of Adonai (not Adoni as Messiah from Psalm 110:1) based on Isaiah 45:23 (LXX). This supports kyrios in 1 Corinthians 8:6 as a divine title.

Conclusion

Scripture explicitly shows the idea of a singular name being addressed to plurality of persons.

Genesis 5:2 Male and female = one Adam, not two Adams

1 Corinthians 8:6 Father and Jesus = one God, one Lord (not two Gods/Lords)

The the Father, Jesus and the Holy Spirit, who all are of same nature, are not three gods but one God just as Adam and Eve, who are of same nature, are not two Adams but one Adam in Genesis 5:2.

Within the One Deity there is a perfection of unity.

Said Jesus :

I and my Father are one (John 10:30 - KJV - εγω και ο πατηρ εν εσμεν)

But Paul also speaks of :

the fullness of God (Ephesians 3:19 - KJV - το πληρωμα του θεου)

If, therefore, Deity is, in essence, fullness ; and if both the Father and the Son share that fulness ; and if there is no limit to that divine fulness . . .

. . and both share that unlimited fulness, in one Holy Spirit . . .

. . . then I believe it becomes clear (to faith, and to faith alone) that Deity exists in a perfection of divine unity. And in that divine nature, dwells the Person of God.

And from within that Deity has come One whom the Father declared :

This is my beloved Son ... (Matthew 3:17 - KJV - εστιν ο υιος μου ο αγαπητος )

who in his turn declared :

I and my Father are one.

As has already been noted, Jesus is "one" with God in the sense of unity. Hence Jesus gives the following prayer:

John 17:20-23 My prayer is not for them alone. I pray also for those who will believe in me through their message, 21 that all of them may be one, Father, just as you are in me and I am in you. May they also be in us so that the world may believe that you have sent me. 22 I have given them the glory that you gave me, that they may be one as we are one— 23 I in them and you in me—so that they may be brought to complete unity. Then the world will know that you sent me and have loved them even as you have loved me.

Furthermore, it is good to note that many have been addressed as God who are not literally God. Only when Jesus is referred to as God do people start demanding that it be taken literally (producing all manner of contradiction). Moses was referred to as God (Exodus 7:1), Angels have been referred to as God (all over the OT, like the angel in the burning bush in Exodus 3), and the Jewish People have been addressed as gods (Psalms 82, John 10:33-36).

Exodus 7:1 Then the Lord said to Moses, “See, I have made you like God to Pharaoh, and your brother Aaron will be your prophet.

John 10:33-36 “We are not stoning you for any good work,” they replied, “but for blasphemy, because you, a mere man, claim to be God.” 34 Jesus answered them, “Is it not written in your Law, ‘I have said you are “gods”’[d]? 35 If he called them ‘gods,’ to whom the word of God came—and Scripture cannot be set aside— 36 what about the one whom the Father set apart as his very own and sent into the world? Why then do you accuse me of blasphemy because I said, ‘I am God’s Son’?

In reality, Jesus never says of himself that he is God, but always maintains that he is the Son of God. It is clear from an honest reading of the scriptures that no one in the scriptures literally considered Jesus to be God Almighty himself, but all maintained that Jesus was the Son thereof, who held a lesser position at the right-hand of God.

Mark 16:19 After the Lord Jesus had spoken to them, he was taken up into heaven and he sat at the right hand of God.

Luke 22:69 But from now on, the Son of Man will be seated at the right hand of the mighty God.”

Acts 2:33 Exalted to the right hand of God, he has received from the Father the promised Holy Spirit and has poured out what you now see and hear.

Romans 8:34 Who then is the one who condemns? No one. Christ Jesus who died—more than that, who was raised to life—is at the right hand of God and is also interceding for us.

Hebrews 1:3 The Son is the radiance of God’s glory and the exact representation of his being, sustaining all things by his powerful word. After he had provided purification for sins, he sat down at the right hand of the Majesty in heaven.

Indeed, there is an abundance of scripture that clearly differentiates and subordinates Jesus to God Almighty. Here's a great verse from Paul:

1 Cor 15:27-28 27 For he “has put everything under his feet.”[c] Now when it says that “everything” has been put under him, it is clear that this does not include God himself, who put everything under Christ. 28 When he has done this, then the Son himself will be made subject to him who put everything under him, so that God may be all in all.

Contrary to the way some may view this question, it is not a "truth" question but a "how" one. "How is it possible," you ask, given that the tiny selection of isolated verses you quote appear at first sight to be contradictory? You are not questioning the truthfulness of those verses but taking them as a given. You are not questioning the truthfulness of the Christian Trinity doctrine (one Being of God subsisting in three uncreated 'persons'), but asking how it is possible that this be so.

It could only be possible if all three who subsist in the one Being of God are uncreated. If one, two, or all three of them had to be created in order to start existing at a given point in time, then the entire Christian faith would be utter nonsense, and the doctrine of the Trinity as gleaned from the Bible would be rubbish. It logically follows that it could only be possible if the Holy Spirit had distinct personality and uncreated existence as does the Word (the Son, who later added human nature to his divine nature in order to become the man, Jesus).

This is where an additional comment is needed, where you stated, "According to these 5 verses, it tells that Jesus was created before all the rest." Your misunderstand of those verses is causing you difficulty in understanding "how it is possible". None of those verses state that the Word (the Son, who later became Jesus) was ever created. This is where you need a grasp of the meaning of the ancient Greek words in the New Testament which have been variously translated into English as 'first-born' and 'only-begotten'. Never in the Bible are words used of Jesus that are to be translated as having had a starting point in time, by being created, or even pro-created. But that is not what you ask about, and I will not go into that here. Fresh questions are required. Let it just be pointed out that those who cannot see the uncreated nature of Christ in scripture, cannot see any possibility of the Trinity doctrine being biblical. Their interpretation of verses about Jesus can be based on misunderstanding what critically important biblical words mean, and may cause them to make serious changes to their own translations of the Bible, to weaken the import of the uncreated nature of Christ, and how "He made everything that was made" (John 1:3), logically meaning that he could not have been made (created) himself. I add this last paragraph to help get you over the 'hump' that prevents you grasping how it is indeed, possible, for the one eternal Being of God to subsist in three equally eternal 'persons'.