Some background regarding Arianism

To answer the question, it is important to understand that Arianism is a Trinitarian heresy that denies the divinity of the Second Person of the Trinity. Specifically, Arius held that the Son, or the Word, was created out of nothing, and that there was a time when He did not exist. The Son was, therefore, the Father’s first creature, but not God in the proper sense. Arius’ views represent the most extreme form of a current called subordinationism.

It is important to recall that Arius never denied the identification of the Word with Jesus Christ. (If anything, he tended to deny the full humanity of Christ, like Apollinarius of Laodicea, but this was not at the heart of the controversy associated with him.)

Biblical passages

In order to refute Arianism, it is sufficient, therefore, to find passages in the Scriptures that show that the Son is fully God, with just as much right to be called “God” as the Father.

St. John’s prologue

The most straightforward passage is the prologue of St. John’s gospel:

Ἐν ἀρχῇ ἦν ὁ λόγος, καὶ ὁ λόγος ἦν πρὸς τὸν θεόν, καὶ θεὸς ἦν ὁ λόγος.

In the beginning was the Word, and the Word was with God, and the Word was God (John 1:1).

I include the original Greek, because the grammar of the text makes a subordinationist interpretation impossible. Notice how the words have a perfect chasmic structure (which is lost in English): the second half of each sentence is always repeated as the first half of the next (ὁ λόγος with λόγος; τὸν θεόν with θεὸς); this was a common rhetorical device in antiquity. St. John must, therefore, be identifying θεὸς with τὸν θεόν, and τὸν θεόν is clearly and unambiguously God.

There is also a reference here to Genesis chapter 1:

In the beginning, God created the heavens and the earth.

If the Word was with God “in the beginning,” then Arius’ claim that there was a time before the Son existed is refuted; moreover, since there was only God “in the beginning,” that Word must be God.

Note that St. John clearly identifies the Word with the Son, and both with Jesus Christ, in the following passage:

And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth. (John 1:14).

“Glory” is something that the Old Testament attributes only to God. Therefore, we may also legitimately use any passages in which Jesus attributes divinity to himself as a refutation of Arianism.

Jesus’ own claims to divinity

Jesus made a number of claims to divinity, although, at least at first, his claims were not often open and explicit, so to as to avoid causing scandal during his ministry.

One of the most explicit claims is found at John 8:58-59:

Jesus said to them, “Truly, truly, I say to you, before Abraham was, I am.” So they picked up stones to throw at him, but Jesus hid himself and went out of the temple.

The key phrase is “I am,” which recalls Exodus 3:14, where God solemnly reveals His holy name:

God said to Moses, “I am who I am.” And he said, “Say this to the people of Israel, ‘I am has sent me to you.’”

Jesus is claiming, at a minimum, to be greater and older than Abraham, but that is not sufficient to arouse such a violent reaction from the Pharisees: they understand him as claiming to be God.

There are also Jesus’ claims to be one with the Father:

[Jesus said,] “I and the Father are one.” The Jews picked up stones again to stone him (John 10:30-31).

“I do not ask for these only, but also for those who will believe in me through their word, that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me” (John 17:20-21; the chapter continues with similar claims).

Jesus also makes an affirmation of His divinity just before his crucifixion, and it is the immediate cause of the charge of blasphemy:

Jesus said to him, “You have said so. But I tell you, from now on you will see the Son of Man seated at the right hand of Power and coming on the clouds of heaven” (Matthew 26:64).

This one might be less obvious, but Jesus is putting together passages from Daniel 7:13-14 and Psalm 110:1. Taken individually, the passages are ambiguous: the “Son of Man” in Daniel 7 seems to refer most directly to the People of Israel. When in Psalm 110, the Lord says, “Sit at my right hand,” the meaning is metaphorical. However, when the passages are taken together and applied to Jesus, we see that Jesus is interpreting the passages to mean that the “Ancient of Days” (who is clearly God) asks the “Son of Man” (in this case Jesus) to sit at his “right hand,” which is a place of equal honor and dignity. Jesus’ persecutors, at any rate, seem to have understood him in this way.

There are also the numerous times, particularly (but not only) in John’s gospel, in which Jesus reveals something important about himself beginning with the words “I am.” (“I am the vine,” “I am the good shepherd,” and so on.)

We could also add that Jesus accepts without protest acts of worship, whereas his disciples in analogous circumstances protested vigorously. For example, compare

Jesus heard that they had cast him [the man born blind] out, and having found him he said, “Do you believe in the Son of Man?” He answered, “And who is he, sir, that I may believe in him?” Jesus said to him, “You have seen him, and it is he who is speaking to you.” He said, “Lord, I believe,” and he worshiped (προσεκύνησεν) him (John 9:35-38).

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And the priest of Zeus, whose temple was at the entrance to the city, brought oxen and garlands to the gates and wanted to offer sacrifice with the crowds. But when the apostles Barnabas and Paul heard of it, they tore their garments and rushed out into the crowd, crying out, “Men, why are you doing these things? We also are men, of like nature with you” (Acts 14:13-15).

In a similar vein, there is also the profession of faith by St. Thomas the Apostle:

Then he said to Thomas, “Put your finger here, and see my hands; and put out your hand, and place it in my side. Do not disbelieve, but believe.” Thomas answered him, “My Lord and my God!” (John 20:27-28).

We could also mention that Jesus performs actions that are explainable only by his Divine Nature: his miracles, the way that he can read people’s intimate thoughts (see, for example, Mark 2:8), and even forgive sins:

And when Jesus saw their faith, he said to the paralytic, “Son, your sins are forgiven.” Now some of the scribes were sitting there, questioning in their hearts, “Why does this man speak like that? He is blaspheming! Who can forgive sins but God alone?” (Mark 2:5-7).

Jesus never challenges the principle that God alone can forgive sins (at least by His own authority, or ἐξουσία), but he does communicate the power to forgive sins:

And when he had said this, he breathed on them and said to them, “Receive the Holy Spirit. If you forgive the sins of any, they are forgiven them; if you withhold forgiveness from any, it is withheld” (John 20:22-23).

It is not a mistake that most of the citations here are from the Gospel of John. St. John seems to have been particularly concerned with emphasizing Jesus’ divinity, perhaps because a current of subordinationism was already present in the early Church.

Passages from other New-Testament writers

It is not only Jesus himself who claimed to be divine, but his disciples taught his divinity in the other New-Testament writings.

Examples from the Pauline epistles

The clearest of these affirmations come from St. Paul. For example, there is the famous Christological hymn from Philippians 2:

Have this mind among yourselves, which is yours in Christ Jesus, who, though he was in the form of God, did not count equality with God a thing to be grasped, but emptied himself, by taking the form of a servant, being born in the likeness of men. And being found in human form, he humbled himself by becoming obedient to the point of death, even death on a cross. Therefore God has highly exalted him and bestowed on him the name that is above every name, so that at the name of Jesus every knee should bow, in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord, to the glory of God the Father (Philippians 2:5-11).

This passage is extremely rich. Among many other things that could be said, I note the following:

• St. Paul says that Jesus Christ is “in the form of God” (ἐν μορφῇ θεοῦ). The expression “form of God” sounds strange to us, but at the time of Paul (especially in a Greek-speaking and fully Hellenized area such as Philippi in Macedonia) the term would have made people think immediately of Aristotle’s μορφή (form) and Plato’s εἶδος (idea or form). Paul is essentially saying that God has the divine μορφή, or nature, in our parlance. In other words, Jesus is fully divine. Notice that later on Paul says that Jesus takes on the “form” (μορφή) of a slave (i.e., of man). So μορφή is Paul’s term for “nature.”
• Jesus “did not count equality with God [literally being equal to God, τὸ εἶναι ἴσα θεῷ] a thing to be grasped,” but neither did he divest himself of that equality. Rather his “emptying himself” consisted in taking on human nature, and, above all, in dying on the Cross.
• Paul says that all creatures (in heaven, on earth, and below the earth, by which Paul means, “the good angels, man, and even the demons”) should “bend the knee.” But this kind of honor is reserved only to God.

There is also a similar passage in the Letter to the Colossians:

He [Jesus Christ] is the image of the invisible God, the firstborn of all creation. For by him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or rulers or authorities—all things were created through him and for him. And he is before all things, and in him all things hold together. And he is the head of the body, the church. He is the beginning, the firstborn from the dead, that in everything he might be preeminent. For in him all the fullness of God was pleased to dwell, and through him to reconcile to himself all things, whether on earth or in heaven, making peace by the blood of his cross (Colossians 1:15-20).

Although Arius left no extant writings, one can imagine that he seized on Colossians 1:15 (that Christ is the “firstborn of all creation”) as “proof” for his doctrine. He would soon have encountered a problem, however, because the “firstborn” is a technical term that specifically implies the right to inherit; in other words, one need not infer from this passage that Jesus is a creature. He could hardly be a creature in any case if he is “before all things, and in him all things hold together. And he is the head of the body, the church”— such things can only be attributed to God. (The phrase “fullness of God,” in fairness, is something of a modern gloss, because the words “of God” do not appear in the original Greek. Still, true ontological fullness, or πλήρωμα, could only dwell in God.) As with the term μορφή from the passage in Philippians, in St. Paul, the term “image” (εἰκών) does not mean “imitation,” but rather “exact likeness.”

Examples from the preachings of the Apostles

Other affirmations of Christ’s divinity can be found in the preachings of the Apostles after Pentecost. For example, there is St. Peter’s discourse at Pentecost:

This Jesus God raised up, and of that we all are witnesses. Being therefore exalted at the right hand of God, and having received from the Father the promise of the Holy Spirit, he has poured out this that you yourselves are seeing and hearing. For David did not ascend into the heavens, but he himself says,

The Lord said to my Lord,
Sit at my right hand,
until I make your enemies your footstool [Psalm 110:1].

Let all the house of Israel therefore know for certain that God has made him both Lord and Christ, this Jesus whom you crucified (Acts 2:32-36).

So Jesus has the prerogative of pouring out the Holy Spirit (v. 33), a power that can attributed only to God. As Jesus himself did during his trial, St. Peter applies Psalm 110 to Jesus, attesting that Jesus sits at the right hand of God, which, in the Jewish mentality, means that he has honor and dignity equal to God.

After Peter cures a lame man, he makes a discourse at the Portico of Solomon. There, he says, in part:

But you denied the Holy and Righteous One, and asked for a murderer to be granted to you, and you killed the Author of life, whom God raised from the dead (Acts 3:14-15).

He is, of course, referring to Jesus’ trial, in which Barabbas was released in his place. Peter affirms not only Jesus’ innocence, but his authority over life itself; this authority can only be God’s. Moreover, the title of “Holy One” in the Old Testament refers to God. (See, for example, Isaiah 30:12.)

We could point to other passages, but these are the clearest.

Conclusion

In summary, we can say that St. John’s gospel makes a very deliberate attempt to demonstrate Jesus’ divinity, and (more centrally to our question) the full divinity of the Word or Son. It is in his gospel that all of the most explicit claims to that divinity can be found.

However, the other Gospels, as well as the other New-Testament writings, make many assertions that imply Jesus’, and hence the Son’s, divinity. Among these, the clearest are St. Paul’s Christological hymns, but there are many other passages that imply, hint at, or otherwise confirm the divinity of Christ.