Muhammad upheld the divinity of Christ but He clearly did not accept that which was voted on and applied to Christ at the Council of Nicaea, when the Trinitarian Doctrine was conceived.
It is said that new religions form after existing religions become corrupted and 622 years after Christ Islam formed. One conclusion being that Islam supplanted Christianity because of the corruption established at Nicaea which basically made the claim that Christ was God, the only path to heaven and that Christianity was the eternal and only true religion and we know how well that worked out for people of indigenous or other mainstream religions.
Muhammad and Christ were equals and both shared a common divinity. So Muhammad was not questioning Christ's divinity, rather He was trying to apply a bit of perspective to Christian claims so that Christians might see Christianity as a religion amongst others and Christ as a Prophet amongst many, not as something singular and exclusively the only true Prophet and religion.
