(Attempting to speak from the Orthodox Christian point of view:) While we know some things due to divine revelation, there's plenty of truth to be discovered through the observation and contemplation of God's creation (both seen [the physical world] and unseen [logic, math, thought itself]). Whatever is true that comes out of Buddhism is true because it comes from God, it just comes by way of extraordinary insight, unaided (as far as we know, in the Christian tradition) by direct communication from God.