The poem begins with a simple question for God: will He forgive the sin that all humans are born into? The speaker acknowledges that this sin isn't his (the speaker's) fault, and that it happened before he was even born. Then, without waiting for an answer, he asks God another question, this time if He will forgive the sins that the speaker continues to practice. The speaker ends the first stanza by telling God that when God has finished forgiving him for these sins, He isn't quite done yet… there are plenty more sins a-comin'.
In the second stanza, the speaker echoes his earlier questions, but this time he's concerned with the sins he's caused others to participate in, not to mention the sins which he was able to briefly avoid for a while before giving in and enjoying. He again reminds God that this isn't the end of sins that need forgiving. They are starting to pile up, in fact.
The final stanza focuses on the speaker's fear of dying before being forgiven for all these sins. He doesn't want to get stuck in Limbo—the place between heaven and hell—and he considers this fear to be another one of his sins: the sin of doubt. Unlike the first two stanzas, though, the final stanza contains a sort-of answer to the speaker's worries. The speaker decides that, as long as God swears that Jesus will still stand as a buffer between man's sins and God, he'll stop being afraid. Sounds like a deal to us.