He who mocks the Infant's Faith Shall be mock'd in Age & Death.
Blake takes a quick detour from politics and economics back to spirituality. If someone makes fun of the simple faith of a child—or, apparently, infant—they're setting themselves up for a rude awakening. Basically: they're going to grow old, and weak, and die.
Of course, that's going to happen to everyone, so how is it a punishment specifically designed for atheists?
Well, in Blake's way of seeing things, it's worse for atheists or skeptics because they presumably don't believe in an eternal soul or eternal world. "Age and Death" sting more because there's nothing beyond them—death gets the final say. But if you believe in an eternal reality, they're not that bad, because there's something beyond them.
We also get another imperfect rhyme here ("Faith" and "Death")—by modern standards, at least.
He who shall teach the Child to Doubt The rotting Grave shall ne'er get out.
This couplet runs on the same theme as the last one—faith vs. non-belief. In this case, Blake is still chiding atheists—but he doesn't actually believe in an eternal hell (which he says numerous times in his notebooks) and doesn't think an atheist won't really be able to get out of the grave.
What Blake is doing is joking about the unbeliever's own beliefs. Someone who doesn't believe in an eternal reality actually does believe that they won't get out of the "rotting Grave," because death is final. Blake is saying that, if you have this skeptical mindset, being trapped eternally in a rotting grave is, as far as your imagination is concerned, a real possibility. Your own belief system is your "punishment," in a way.