It this spirit, I am going to consider Gospel messages in limited formats. For example, what would you say given a limit of, say, one word!
This one is actually easy, because I pull it straight from the Bible:
"Thus saith the Lord GOD; Repent, and turn [yourselves] from your idols; and turn away your faces from all your abominations." (Ezekiel 14:6)
"Repent ye: for the kingdom of heaven is at hand." (Matthew 3:2, that's John the Baptist)
"From that time Jesus began to preach, and to say, Repent: for the kingdom of heaven is at hand." (Matthew 4:17)