OK, so mercy is not the right word. But I don't quite know which word to use in describing what I'm talking about.
I start off with puzzlement. I'm puzzled by those who profess to hate/dislike/be offended by the God of the Bible (and there is no other God: any other representations are to a greater or lesser degree idols) and yet who still want to end up in heaven.
We don't know a lot about heaven. Or hell. We know that heaven will involve the praise of God, that we will sing there (one of the few skills from this life that we know will carry over to the next), we know that we will be with God and His saints -- but beyond that, we don't know a lot of details.
The Bible just doesn't say a lot about heaven. Primarily because, I suspect, most of what heaven will be like would be pretty incomprehensible to us. It would be like if I could (I'm stealing this analogy) take a not-yet-born infant and try to describe what the world is like: beautiful sunsets, the sound of an opera, the smell of a rose, the grace of friendships, the taste of fresh bread. All of this would make no sense at all to our hypothetical infant, because she would have nothing to understand these descriptions. So heaven: we have only the most limited ability to understand what joys will be ours. So the Holy Spirit through the biblical writers just doesn't bother. We'll know soon enough.
But we all encounter folks who have no interest in church, dislike talking about God, do not pray to Him, and do not hear His Word. And yet these same people will often complain bitterly about a suggestion that they will not be in Heaven.
The biggest question is why they would want to be in Heaven in the first place. They have no interest in even the little that we know about Heaven; why would they want to be there?
God could have made us robotoids, beings incapable of turning away, beings who loved Him because we had no other options. He did not make us so. Matt. 23.37 is sad evidence that God permits us to turn aside His love.
As we don't know much about Heaven, so we know little about hell, except that it is a place of torment and sadness. But perhaps hell is also the ultimate statement of God's respect (I can't think of a better word) for humankind. Because we as a race are given the ability to turn aside from His love, and those who turn aside from that love go into hell. Not because God hates them (John 3 forever refutes that dreadful idea) but because will not force these people into a Heaven that they don't want, a Heaven where everything about God that they have despised would be forced in their face for eternity.
Hell is a sad and terrible place and Jesus -- who speaks of Hell more than anyone else in the scriptures -- never speaks of it except in the saddest of tones. Sad because those poor creatures who end up there perhaps spend eternity knowing why they are there, and despising the Creator who made them capable of joy -- joy which they away from -- here on earth, and there in eternity.