It is a rather deep theological point, but the Bible says that "the gifts and the calling of God are irrevocable." That is, once God chooses someone as His messenger, that's it. The messenger can screw up and make a mockery of himself and God, maybe even to the point that God no longer uses him; but God's calling is still on the person.
Read about David and Saul sometime (I Samuel). It is quite instructive.