Very nice!

A couple of comments.

The phrases are well crafted and have a nice "bloom" to them, but the rests between each phrase is a bit repetitive. The piece would seem less disjointed if you dovetail some of the phrases together (as you do with the S and T in measures 4 and 9). The repetitive feeling is also not helped by so many of the phrases ending on the same chord (the "dominant.")

The preponderance of four-bar phrases gets repetitive as well, which is only highlighted by the rests between each one. The second and last phrases are a welcome relief from this pattern.

Not to be too pedantic, I see a few "faux-pas" according to the strict rules of voice leading: parallel octaves, fifths, and a "beat to beat" parallel (mm. 11-12, S and B), though most of these (with the exception of mm. 28-29 in the tenor and bass) are not that noticeable. Also, there are a few places where all the voices move in the same direction (though not in parallel motion) which is not the best: mm. 24, 38.

One final nit-pick: I would place the penultimate syllable of eleison only on the final note preceding the "-son." With the accent on the "-le-", it sounds odd to my ear. There are also some spots with repeated notes where I wonder whether you misplaced the syllable by mistake or left out a tie (as you evidently did at mm. 26-27 in the S): m. 5, S; m. 10, T; m. 34, T; m. 37, B.

But these are really minor concerns. The individual parts look quite singable and the part writing is varied, tight, and effective - writing in three parts is not easy! I look forward to seeing the other movements.

This is very nice.

I concur with Rich about the continuous four-bar phrases. Pulling a note or two forward, and delaying an entrance in one or two of the three parts could allay some of the tediousness. E.g. mm17-18: if you pull the tenor entrance forward so that it begins on beat 3 of m 17, and tie it over to beat one of m 18 and continue as written, it will create a nice hemiola in the tenor. In m. 14, bass, you could put a rest in place of the low Bb, and change the underlay of "Christe e-" to fit the remaining quarter and two eighths.

I would also change the underlay of Christe to match the Graduale: Chri-ste, rather than Chris-te.