The second-century Greek philosopher Celsus captures well just how upside-down the Kingdom of God is—and just how confusing that can seem to unbelievers. In an attack on followers of Christ, he writes:

Those who summon people to the other mysteries [i.e. other religions] make this preliminary proclamation: "Whosoever has pure hands and a wise tongue." And again, others say, "Whosoever is pure from all defilement, and whose soul knows nothing of evil, and who has lived well and righteously." Such are the preliminary exhortations of those who promise purification from sins.

But let us hear what folk these Christians call. "Whosoever is a sinner," they say. "Whosoever is unwise, whosoever is a child, and, in a word, whosoever is a wretch, the kingdom of God will receive him." Do you not say that a sinner is he who is dishonest, a thief, a burglar, a poisoner, a sacrilegious fellow, and a grave-robber? What others would a robber invite and call? Why on earth this preference for sinners?