The key word is believe. The doctrine of the Incarnation is a belief, not a piece of evidence. No one can prove to you that Jesus is "one in being with the Father," and if they say they can, you ought to cross to the other side of the street.

The term "Son of God" is key to the Christian theology of Incarnation. In Hebrew scripture, "son of God" denotes a person with a special relationship to God. In the New Testament the term describes the unique relationship of Jesus to God. In the Jewish sense, then, angels are sons of God. So are the whole people of Israel and the king of the nation. Finally, Jews post-biblically began to refer to the anticipated Messiah as son of God. None of these Jewish usages implied a divine nature, only a privileged relationship.

In Christian usage the title is first applied to Jesus because he saw himself that way. God is his Father: It's repeated often enough that we might just as easily say Jesus saw himself as God's son. But did he mean "son" in the Jewish sense or the later Christian doctrine? We don't know. Clearly the gospel writers and Saint Paul used the title after Jesus' resurrection appearances to say something more about Jesus than anyone had claimed before.

Here's a brief rundown of how the term evolved for early Christians. First, they understood that Jesus views himself as the "Son in whom [God] is well pleased"—as testified by a heavenly voice at the accounts of his baptism and Transfiguration.

Next, they saw Jesus as the anticipated Messiah; as good Jews they used "son of God" for that awaited figure.

Third, as the church moved into the Gentile world, concepts like "messiah" began to lose their meaning. The world outside Judaism wasn't waiting for a saving hero. But the pagan world did employ "son of God" to refer to heavenly beings of various sorts. The term was a better fit to describe what Christians meant by Jesus.

As the church fathers mined scripture in search of understanding, they drew together traditions of prophecy, Wisdom, and Divine Word (the Logos) to see Jesus as sharing in the divinity of God both before his human birth and afterwards. At the Council of Nicaea in 325 Jesus was declared begotten of God before time began and "one in being with the Father."

Scripture
• Genesis 6:2; Exodus 4:22; Deuteronomy 32:8; 2 Samuel 7:14; Job 1:6; Psalm 2:7; Jeremiah 31:9; Hosea 11:1; Matthew 5:45; 7:21;11:25-27; 16:16; 26:63; Mark 1:1; 14:36, 61; Luke 10:21-22; John 1:1-18; 11:27; 20:31; Romans 1:3-4; Galatians 4:6; 1 Thessalonians 1:9-10; Hebrews 1:2-4

Online
• Declaration on the Relation of the Church to Non-Christian Religions (Nostra Aetate), Second Vatican Council, 1965 (especially no. 4)

Books
• The Reality of Jesus: An Essay on Christology by Dermot A. Lane (Paulist Press, 1977)
• Tradition and Incarnation: Foundations of Christian Theology by William L. Portier (Paulist Press, 1994)