Four short lines, and almost nothing else: that's all the poem gives us. The title, "him.", points at a single person, and the poem sums up what he meant in one idea. With him, nothing felt like a sin. The speaker seems to be describing a relationship, probably romantic, though the poem never says so outright, in which guilt or shame simply dropped away.
What makes the line interesting is how much it leaves unsaid. We don't learn what the "sins" might be, whether they're things the speaker had been taught to feel bad about or just ordinary desires. The poem suggests that the speaker usually carries that weight, and that this person lifted it.
The mood is quiet, warm, and a little wistful. The line breaks slow the reading down, so "Nothing" and "Sin" each land on their own. The past tense in "felt" also hints that this freedom may belong to memory, which adds a touch of longing.