Did you use MuseScore to typeset this?

I just wrote a detailed examination of this, and lost it when the system lost my sign-in. It may have been a sign. But here's the short form:
1. Get a reference on music editing and typography, to clean up some of the presentation problems (rest and accidental usage)
2. Consider the study of Renaissance or species counterpoint, to clean up some of the voice-leading and harmonic issues (6/4 usage, parallels, strong harmonic rhythm)
3. Above all, KEEP WRITING! There are a number of good features here, in the ideas and textures and their deployment. You have good musical instincts, and it's pretty impressive for a first piece.

Stephanie-
The stuff you learned in Harmony 1 should be enough, as that's a kind of reduction of voice leading concepts...but you may want to review it (and 4 part chorales are the things that separate music majors from wanted-to-be music majors, so you may have suppressed the traumatic memories, lol!). I'm fond of the old A. Tilman Merritt book, but that's a free approach. If you want to do species, Fux Gradus ad Parnassum is the fountainhead and is available in a cheap English translation. You might also enjoy nosing around in Pts. 2 (improvised counterpoint) and 3 (composition) of Thomas Morley's Plaine and Easie Introduction to Practical Music, in which the snotty older brother gets his comeuppance for having studied with "Master Bold". What's important in all that is to not only learn "the rules" but why they exist and what they accomplish, so that, when the musical context requires, you can break them with impunity.

A lot of harmony is in thinking about weight: chord changes with one new tone vs 2 or 3 new tones, diatonic changes vs changes with notes outside the key, the stretch between the simplest consonance and the crunchiest dissonance (downshifting the dissonance level..going from stacked M7ths to an open 5th is a bit like dropping from 4th to 1st gear without any time in between). Ultimately, "if it sounds good, it is good", but one can develop one's own taste, and also deal with what sounds good to more people.

You may want to watch where your voices lie. Your tenor part tends to be in the baritone range, which can be easier for amateur singers, but can also sound weak or "muddy" as well as give a rather heavy sound the the piece when combined with the bass part. Each voice part has particular characteristics in various places in their ranges - a "middle C" sung by the sopranos will sound very different from one sung by the basses, or even the altos, something that is not captured by the midi rendition or by playing the reduction at the piano.

Also watch the spacing between voices. A good rule of thumb is to avoid a space of more than an octave between adjacent voices, with the exception of tenor and bass. (Perhaps the tendency to have the tenor and bass close to one another comes from composing at the piano? I don't know.) Measures 25ff., for example, can feel heavy in the tenor and bass while at the same time sounding "screechy" in the upper parts, unless you are dealing with better singers. At measures 36ff., the altos are very exposed near the top of their range, while the tenors are near the bottom of their range - probably not the best combination. (In measure 56 the soprano part is more in their range.) See also measures 22ff. and 40ff. Of course you will see the great masters defying these guidelines, but usually sparingly and for a particular effect. So these are not hard and fast "rules," just things to keep in mind. Writing music that is gratifying to the various voices never hurt anyone!

A few more parallels, presuming you want to keep to traditional voice leading:
Parallel 5ths at measures 14-15 (A-T); measures 75-76 (S-B)
Parallel 5ths and octaves at measures 49-50 and measure 53.

I agree with Jeffrey - this is an impressive first piece! You have some nice harmonic variety (the bane of most beginners), especially at measures 45ff. which you could develop even more - it seems to run out of steam too early. Rhythmically it's perhaps a bit stolid, but the recurring motif on "Deus" is effective, particularly at the end of the build up from measures 17 to 25. The subsequent relaxation to measure 30 is also well done. Nice work!

And here's one of that same passage in 4 voices.

Thank you all so much for your critiques. They are invaluable!

Stephanie:

I listened to the MIDI file. You are off to a good start. Listen to hours and hours of polyphony... NOTHING can replace that experience. Copy some of the works of Pachelbel, and the Renaissance polyphony masters by hand. Analyze them. Imitate what they include in their comps. Write a lot of SATB homophonic harmonies to familiar melodies. These are all things that will keep you going in the right direction.

God Bless!

fk