Q: Was the divinity of Christ really decided upon by a close vote at the Council of Nicea? A: No. The Council of Nicea was called to discuss several matters - one being false teaching in this area. The divinity of Christ was nearly a unanamous concensus.
The Da Vinci Code falsely states that "Jesus was viewed by his followers as a mortal prophet" (Brown, D., 2004, p. 240) and that "Jesus' establishment as 'the Son of God' was officially proposed and voted on by the Council of Nicea...a relatively close vote at that" (Brown, D., 2004, p. 241).
Councils were not uncommon to the early Christians. They were important to discuss doctrinal matters and prevent the spread of false teaching (Garlow & Jones, 2005, p.49). In AD 318 Arius was spreading a false teaching that Jesus was a created being - just like other humans. He was declared a heretic by Bishop Alexander of Alexandria in AD 321 (Thomas, J. 2006).
Arius moved to Palestine and began sending letters to churches promoting Jesus as a created being. The debate grew and to settle the matter, emperor Constantine I called together more than 300 bishops from around the Roman empire (Garlow & Jones, 2005, p.49). The bishops met in AD 325, in the town of Nicea and only two bishops voted in favor of Arius' position (Thomas, J. 2006).
This is hardly a close vote as the Da Vinci Code portrays it. Furthermore, the council of Nicea did not meet to propose the establishment of Jesus as 'the Son of God', but rather to dispel false teaching otherwise. Other than a handful of people at the time who were apparently stirring up trouble, the majority of people did not need to vote on Jesus' divinity - they already knew he was divine.
References:
Brown, D. (2004). The Da Vinci Code, Special Illustrated Edition. (p. 241). New York, NY: Doubleday.
Garlow, J. & Jones, P. (2005). Cracking Da Vinci's Code. (p. 49). Colorado Springs, CO: Victor.
Thomas, J. (2006). The Da Vinci Code. Hatfield, PA: Biblical Seminary.