Descarte's believed for different reasons...

It is possible to believe in the same things as others, but for different reasons. Descarte believed in God, but chose to back up his belief with something other than faith. The Catholic Church, which he was devoted to, based much of their belief in God on faith alone. Descarte decided that faith was not enough for him and chose to validate his belief based on reason and truth. He did this by doubting and then rebuilding his belief based on what he knew to be certain. Just because he chose to believe in God based on reasons different than those of the church do not make his belief right... or wrong.