21 Now when all the people were baptized, and when Jesus also had been baptized and was praying, the heavens were opened, 22 and the Holy Spirit descended on him in bodily form, like a dove; and a voice came from heaven, “You are my beloved Son; with you I am well pleased.”
The picture of Jesus at his baptism is one of the sweetest and simplest arguments for the triune nature of God that one can find in the Scriptures. Though there are many groups, Oneness Pentecostals and Muslims for example, who would argue that the Bible does not teach the doctrine of the trinity, this simple snapshot shows us all three persons of the one Holy God in one frame.
Look at the picture spelled out in the words of the verses above. God the Father’s voice is resounding, declaring Jesus to be his Son. Jesus, God the Son, is center-stage, coming up out of the water. God the Holy Spirit is descending upon Jesus visibly.
There is no modalism here. God the Father did not become the Son. God the Son did not become the Holy Spirit. No, all three persons are present in one biblical event.
Is there polytheism here? No, because Scripture does not contradict itself. There is only one God. Yet, in an amazing way that we cannot grasp with our finite minds, that one God is revealed in three persons. Each person is distinct and yet there is still only one God.
From the Second London Baptist Confession of 1689:
Chapter 2: Of God and of the Holy Trinity
1._____The Lord our God is but one only living and true God; whose subsistence is in and of himself, infinite in being and perfection; whose essence cannot be comprehended by any but himself; a most pure spirit, invisible, without body, parts, or passions, who only hath immortality, dwelling in the light which no man can approach unto; who is immutable, immense, eternal, incomprehensible, almighty, every way infinite, most holy, most wise, most free, most absolute; working all things according to the counsel of his own immutable and most righteous will for his own glory; most loving, gracious, merciful, long-suffering, abundant in goodness and truth, forgiving iniquity, transgression, and sin; the rewarder of them that diligently seek him, and withal most just and terrible in his judgments, hating all sin, and who will by no means clear the guilty.
3._____ In this divine and infinite Being there are three subsistences, the Father, the Word or Son, and Holy Spirit, of one substance, power, and eternity, each having the whole divine essence, yet the essence undivided: the Father is of none, neither begotten nor proceeding; the Son is eternally begotten of the Father; the Holy Spirit proceeding from the Father and the Son; all infinite, without beginning, therefore but one God, who is not to be divided in nature and being, but distinguished by several peculiar relative properties and personal relations; which doctrine of the Trinity is the foundation of all our communion with God, and comfortable dependence on him.