As Christians, we belive that Jesus is God. We don't believe that he is merely a god (cf. Mormons) or that he simply has divine qualities. We beleive that he is the God who created the universe.

He 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. - Colossians 1:15-17

He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power. - Hebrews 1:3

We believe that he is rightly identified with Yahweh God of the Old Testemant, "Yahweh" being the personal name with which God revealed himself to his people in the Old Testament (see Exodus 3:14-15).

It's mysterious, to be sure, but the Jewish leaders of Jesus' time wanted to kill him for this very claim. 

It is not for a good work that we are going to stone you but for blasphemy, because you, being a man, make yourself God. - John.10:33

Time and again, New Testament references to Yahweh God of the OT are applied to Jesus. Here's a sampling:

• Matthew 3:3 / Isaiah 40:3
• Matthew 21:16 / Psalm 8:2
• Romans 10:9-13 / Joel 2:32
• Romans 14:11 / Isaiah 45:23
• Philippians 2:10-11 / Isaiah 45:23
• 1Corinthians 1:31 / Jeremiah 9:24
• Ephesians 4:8-10 / Psalm 68:18
• Titus 2:13 / Psalm 130:8
• Hebrews 1:10 / Psalm 102:25-26
• Revelation 22:12 / Isaiah 40:10