The fact that someone quotes from Scripture doesn’t necessarily make him a Christian. Certainly Lincoln, like everybody in his time, was influenced by Christianity and the Bible, it was inevitable. The question is, was Lincoln personally a Christian believer?
Lincoln went ahead with the civil war because Jesus, in The Bible, said that a house divided cannot stand. For him to use this saying from Jesus to go ahead with the Civil War had to mean that he had the utmost of respect for The Bible and the words of Christ.