Introduction to Islam:The Giving of the Qur'an and the State of Church History in AD 610
When Muhammad received the Qur'an in A.D. 610, the Church had already settled issues regarding the Trinity (as seen in the Athanasian Creed, 6th c.), Christology (as seen in the Definition of Chalcedon, AD 451), and- largely- the canoncity of Scripture (as seen in codices such as Vaticanus and Sinaiticus, 4th c.).
In the early seventh century, there were also some specific errors within the Christian community. By A.D. 610, the Veneration of Mary had also increased to the point that if Muhammad had visited a church, he would have likely seen icons of Mary. Also, false Gospel accounts, like the Gnostic-tinged Infancy Gospel of Thomas, were still influential among some who claimed to be Christians. Both of these errors- the Veneration of Mary and the use of spurious "Gospels"- would impact the teachings of the Qur'an concerning Christianity.