He who mocks the Infant's Faith Shall be mock'd in Age & Death. He who shall teach the Child to Doubt The rotting Grave shall ne'er get out. He who respects the Infant's faith Triumphs over Hell & Death. (85-90)
Blake clearly cares a lot about "the infant's faith"—people who damage it are quite bad, in his view, and people who respect it are headed in the right direction. Blake's idea seems to be that faith is part of human nature—it's something a baby or small child just has. So, someone who damages that, who tries to take this basic trust and bend it toward skepticism, is effectively ruining humanity—making humans less human (according to Blake).
Without a belief in an eternal reality, says Blake, you've really got nothing to look forward to except for old age, sickness, and death. That's what the "mock'd in Age & Death" part and the "The Rotting Grave shall ne'er get out" part are all about. But, if you respect the infant's faith, then you triumph over death, since you're headed toward eternity.