Why I'm Glad Christ's Enemies Don't Know About This Verse

Rev. 22:11a, "He that is unjust, let him be unjust still: and he which is filthy, let him be filthy still..."

If unbelievers knew about this verse they would use it as an excuse to continue in sin. Obviously we know that is not the meaning, but what DOES the verse mean?

And what about Rev. 22:12, "...my reward is with me, to give every man according as his work shall be."

Does this verse teach salvation by works? If not, what DOES it teach?