"Born Again" doesn't actually mean anything.
I think it probably means something, at least to the people who use the phrase seriously. In the gospel according to St. John, Jesus said something about being born again. I don't read Greek, though, so I don't actually know what John said, just what translation say he said. There are other English translations that use the phrase "born from above" instead of "born again." I do think that the observant evangelical Christians, who very often use the Jamesean English translation of 1609, do associate a real meaning with the phrase.
Or do you mean that it hasn't any meaning for The Donald? That I can agree with.
