If Jesus is God, then why in John 20:17 did he say “I am ascending to my Father and your Father, to my God and your God.”? This tells us that Jesus as well as us have a common God.

This is a difficult passage to interpret as long as there is a false understanding of the Trinity and Hypostatic Union present. The Trinity is the Christian teaching that there is one God (Deut. 6:4; Isa. 45:5, 14, 18, 21-22) who exists in three persons (Gen. 1:26-27; Amos 4:11; Matt. 28:19; 2 Cor. 13:14). The Hypostatic Union defines the two natures of Jesus Christ which are human and divine (Col. 2:9; Phil. 2:5-8;  Tit. 2:13; 1 John 5:20).

Jesus is the second person of the Trinity. Scripture tells us that it is the Father who sent Jesus (John 6:44; 8:18) to do His will (John 6:38). Almost every place where Jesus is talking about or to God, it is in reference to the Father, the first person of the Trinity. In this verse when Jesus uses the term Father and God, he is talking about the same person. We can illustrate it as though we were speaking of a brother, “he’s Timothy to me, he’s Timothy to you. He’s my brother, he’s your brother.”

Seeing as Jesus used the two terms ‘Father’ and ‘God’ to refer to the same person throughout his ministry, there is again nothing wrong with doing it here.