Eliot is making a reference to the prologue of the Canterbury Tales when he calls April the cruelest month. Chaucer describes April in terms describing rebirth....the characters in the prologue are about to set off for a pilgrimage. There is a deep sense of faith in the prologue to the Canterbury Tales, but not in Eliot's poem. The comment reflects his lack of faith...or so scholars tell me. There has been so much written on these opening lines so I imagine you could easily find something in writing that could explain it more eloquently.