In Islam, are Christians considered to be idolaters? If it isn't considered idolatry, my question is why? Christians believe a person (Jesus) is God.
There are Christians who - reading the Word of God in the Bible - came to the same Message as we find it in the Quran. There is a quite explicit passage in the Bible, and many other indications that Jesus clearly said that he is not God.
Gospel of Mark, 10:17-18
And as he was setting out on his journey, a man ran up and knelt before him, and asked him, "Holy1 Teacher, what must I do to inherit eternal life?" And Jesus said to him, "Why do you call me holy? No one is holy but God alone.
Witnesses of Jehova and Unitarians reject the trinity dogma and pray to God only.
Reformed and Presbytarian Churches only accept the teachings of the Bible. Members are allowed to accept the trinity dogma but it is not part of reformed theology. Images in churches and praying to persons other than God are forbidden. (many other Protestant churches do pray to Jesus).
The Oriental Church was originally also rather Unitarian; many unitarian Christians have supported the Prophet and accepted Islam. The acceptance of Christians is also to be seen on this background.
Other Churches do not understand Jesus as a deity other than God, but that Jesus and the Holy Spirit are manifestations of the same God. This teaching is clearly told to be wrong (shirk) in the Quran but somehow not estimated the same as worshipping other deities.