Is Freemasonry a religion? What do masons believe?
Masonic organizations state up front that they are not a religion, but instead encourage their members to grow in whatever faith they have. However, the tenets of Masonry include some basic theology, which is not always compatible with Christianity, despite what Masons say. Masons are required to be theists. They also implicitly (and perhaps explicitly) affirm the idea of religious pluralism by prohibiting religious debate and disagreement among members, thus preventing Christians from questioning others' false beliefs. Catholics in particular should note that becoming a Mason is grounds for excommunication (see the article on Masonry in the Catholic Encyclopedia).
There is also some controversy about the teachings of the afterlife contained in some Masonic rituals. Masons say that Masonry doesn't teach anything about how to get into heaven, including whether or not good works are a requirement, though some Christians disagree with this. (Eschatology by Edward King gives a pro-Masonic discussion of this point.)
In short, while Masonry may not be a religion, its prohibition of evangelizing other Masons is directly at odds with the goals of Christianity. For more on this, see How can Christians say Jesus is the only way to God?