Jesus is both God and man
References: Genesis 3:22, John 1:14, Colossians 2:9, and I John 4:1-4.
Excellent Site for further study: Christian Apologetics and Research Ministry
To me, John 1:1-5 are the most awesome verses that describes Jesus' essence:
1 In the beginning was the Word, and the Word was with God, and the Word was God.
2 He was in the beginning with God.
3 All thing were made through Him, and without Him nothing was made that was made. 4 In Him was life, and the life was the light of men.
5 And the light shines in the darkness, and the darkness did not comprehend it.
This is an ESSENTIAL belief that if denied, then it seriously questions one's Salvation. It is this essence of Christ that allows Jesus' Death on the Cross and Resurrection to Atone for our Sins. It He was/is not God, then the Cross was just the cross, another crucifixion by the Romans.
Denial of this Number One essential shakes the entire foundation of Christianity, and the authenticity of the Bible.
Jehovah's Witness and Christian Science both deny this essential. Of course all other religions such as Islam, reject this belief.