First hearting: very nice indeed! I'll try to give it more attention as soon as possible. Thank you for posting this.

TRR,
Upon first hearing, I think your heart is in the right place! I can also see that you have ideas and inspirations (coming from the Holy Spirit!) and are starting to translate it into music.

I would suggest looking at the great composers that enriched the Church’s treasury of sacred music:
Palestrina, Victoria, Guerrero, Bruckner, and contemporary masters such as Kevin Allen, Frank La Rocca and even some composers on this forum (CHGiffen, Serviam Scores, and others)!
I’m sure you’ve heard their works before.

As you delve into the wonderful art of composing for the Sacred Liturgy, there are two aspects that I consider to be essential:
The spiritual aspect and the technical aspect. You need both to compose good liturgical music and as I said you’re headed in the right direction!

I’ll probably share it sometime but I typed out a document with reminders including with a brief prayer asking the Lord to guide my hand to put down what He has inspired from above, a reminder to pray with and consider deeply the text that I am about to set, etc.

As an aspiring composer myself, the technical aspect is one part that I need to master but is almost equally important. A basic understanding of harmony and counterpoint will boost your ability and confidence. Perhaps others could recommend good texts to study such as Fux’s book on counterpoint.

Sorry for the longer post but I hope this helps!

I like where you're trying to go with the sound of this piece. There are a lot of issues with how you're getting there. Your base note value (1:1 in Fuxian terms) seems to be the half note. So in m.14 you're already running into issue with the alto d being a nonharmonic tone. Especially since you're just beginning the polyphony, you can fake that it's a passing tone in a glacially slow 2:1, but then it has to pass TO somewhere. That would be C, but then that's also a dissonance needing resolution. Your best bet in 15 is to just keep the D in the alto, which also gets rid of your direct octave to a doubled leading tone across the barline. Then in 16 you have parallel octaves between alto and bass, which sounds clunky because it's as if one of your voices just dropped out.

My sense here is that you'd like a richer sound with more seventh chords. You can have consonant 7ths, but singers will have an easier time if they're prepared. And they need to be the prevailing sonority until cadence points, in the same way that 3rds prevail in common practice music. And that still doesn't get you free from the harmonic-control and voice-independence elements of classical counterpoint. My advice would be to make a study of counterpoint: Renaissance or Baroque, free or species. I grew up free, but lean more toward species now. Even Bach-chorale style harmony will teach the principles in embryo. We look at the history of 20th century music and get antinomian, but Schoenberg knew his counterpoint, I assure you.

There are also some nonstandard notation practices which I realize are required to get what you want out of cantamus. You may want to edit this for real singers.

How did Bach manage with 20 kids?
1. He learned the basics of his craft before marriage.
2. He employed his kids as copyists.
3. The older kids helped care for the younger.

Too late for #1, you've got a decade or so before #2 can kick in (and in these days of notation software, it's not a factor), and likewise for #3. So yes, a challenging time.

Free counterpoint (Fux' 5th species) is "real music", whereas species counterpoint is kind of an abstraction using a cantus firmus. The approach of Tilman Merritt (and Kent Kennan to some extent) would be to learn to ride without the training wheels. But the problem always is to keep the harmonic and linear elements in balance. If your lines generate the harmony, things sound chaotic; if harmony generates line, they're boring. A cantus firmus forces a harmonic rhythm. One doesn't want to stay there: of the 16th-c. cantus firmus Proper settings, Isaac's are the best of a bad lot. But it's a useful stage to pass through.

Do you recommend that beginner composers study counterpoint or harmony first?

I can easily compose melodies in a neo-Gregorian/chant style and melodies with a slightly more modern rhythm and style, but my attempts at polyphony have so far been just by ear (I compose a melody, record audio of it and, listening to it, I improvise other voices that I'm recording overlapping, the score comes later).

You're doing awesome as a composer starting out! The only thing I had an issue with was the tempo changes: I'm pretty confident you can find unique rhythms (quintuplets, septuplets?) that can give you the same duration of the notes in measures where the tempo changes occur.