I think this poem is pretty self explanatory. Please Lord, forgive me for not going to church since we die anyways. We'll be together forever.
However, this poem can be read further and deeper than just the surface level forgiveness. But should I? Why not.
"I didn't go to church today, / I trust the Lord to understand," and why wouldn't the Lord understand. Isn't forgiveness of sin part of the doctrine. The speaker must have a good reason though: family emergency, personal crisis, maybe perhaps even something as plausible as his car breaking down.
But no, the next few lines isn't an excuse, it's what actually happened, "The surf was swirling blue and white, / The children swirling on the sand." Yes, if I want to go deep enough I can say this is a biblical allusion to Moses trials of parting the sea and he and his children wandering the desert for 40 year before finding their place.
But no, let's not go there, why? This poem's tone is jovial, not to be over thought because, "He knows, He knows how brief my stay, / How brief this spell of summer weather,"
Up front, this is all forgiven anyways, so why hide it. Why look deeper at reasoning or purpose when it's all planned out.
And the speaker is genuine with this emotion, "He knows when I am said and done / We'll have plenty of time together." So what's one day? What's that one line to say you're a believer or not and who is to judge. Not man. The lord knows.