Ordained means invested with the authority to act as a priest. Once you are ordained as a priest, you are ready to take over that little country church and start tending to the flock.
Ordained comes from the root meaning "order," and when you are ordained, you're brought into the religious order, or group of church leaders. We also use ordained for declarations. If it has been ordained that all people must now wear plastic birds in their hair, then that new fashion command has been declared by an authority. Sometimes people talk about fate as "what has been ordained."