Ten syllables, four lines, and a title that has to be read as part of the poem. In that space, "With/in" describes a dwelling made to suit sin and night, and then, in a single word, empties it. What matters is the way the poem arranges its fragments so that the reader has to decide what the emptiness means, whether relief or loss.
Start with the title. The slash splits "within" into "with" and "in," two small prepositions that point in different directions. "With" suggests company, and "in" suggests enclosure. Together they describe an interior shared with something else. The slash also works like a threshold, making us step through the word before we enter the poem. There is a quieter echo as well. "In" is hidden inside "sin," so the title's last syllable returns, with a letter in front of it, at the end of the second line. The poem rhymes with its own title, and the rhyme says that sin is something found within.
"Fitting:" is the first word, and it can be read two ways. As an adjective it means appropriate, as in "it is fitting that..." As a participle it names an action, the joiner fitting a door to its frame or the tailor fitting a coat. The colon that follows promises an explanation, and what comes after is two matched pairs: "house to sin, home to night." The grammar is loose enough that "to" can mean "suited to," "given to," or "assigned to." No subject and no main verb tell us who is doing the fitting, so the poem has the feel of a definition or an inscription, a statement without a speaker.
The pairs are not identical, though. Sin gets a house and night gets a home. A house is a structure, a container with walls, while a home is somewhere you belong. The poem hands sin the cold word and night the warm one, and the effect is to separate two things we might expect to be lumped together. Sin is lodged, a tenant in a building, while night seems native to the place. Darkness is treated as something that belongs, and wrongdoing as something that merely occupies. The title makes it tempting to read the house as a self, a body or a mind, but that is an inference from the word "within" rather than something the poem says.
The sound and shape support this. The syllable count runs 2-3-3-2, a symmetry in which the two central lines mirror each other and the single words frame them. Both middle lines follow the same stress pattern, with the beat falling on the first and last syllables: HOUSE to SIN, HOME to NIGHT. The repeated *h* ties "house" to "home," which makes their difference in meaning more noticeable. The vowels are tight and short through "Fitting," "with," "in" and "sin," like a snug joint. Then "night" opens into a long, ringing vowel, the one place where the poem lets its mouth open. Every line ends on punctuation (a colon, a comma, a full stop), so the reader never glides on and instead halts four times, like someone walking through rooms one at a time.
Then comes "Emptied." It is a one-word sentence and a trochee that falls away at its end, and it is the only passive construction in the poem. It names a result but no cause. Who or what did the emptying? Perhaps sin and night have been cleared out and the house is clean. Perhaps everything has been drained, including the things that fitted it so well. The word also sets itself against "Fitting." A place that was shaped so exactly to its contents now has nothing to contain, and the snug joint has become a hollow. The white space under the final line does some of the work, since the page itself is left mostly empty.
That is why so short a poem repays attention. It sets up an expectation with the title, "within," and the expectation is that we will be shown what lives inside. Instead we are told how it was fitted and then that it is gone. The poem withholds both the cause and the feeling, and it won't say whether the emptied house is cleansed or bereft. Each reader has to supply that verdict, which means that finishing the poem and reading it are not quite the same thing.