[4:171] O People of the Scripture, do not go to extremes, in your religion and do not say about God except the truth: the Messiah, Jesus the son of Mary, was only the Messenger of God, and His Word which He cast to Mary, and a spirit from Him. So believe in God and His messengers, and do not say, 'Three'. Refrain, it is better for you. Verily, God is but One God. Glory be to Him, that He should have a son! To Him belongs all that is in the heavens and in the earth. God suffices as a Guardian.
Their going to extremes in their religion was their following their own opinion, since they described what they worshipped in the likeness of created beings. There was also their being contradictory, since they said one is three and three is one. The perseverance in what is false only increases what is false.