I'd like to recommend to you the piece "Infelix ego" by William Byrd, especially as sung by Oxford Camerata (yes, it's the album I recommended in the post below this).
William Byrd was one of the foremost composers of Renaissance sacred music. Byrd and his mentor, Thomas Tallis, are notable not merely in that they are the two foremost English composers in the Renaissance style, but that they composed sacred music during the English Reformation and managed to stay on the good side of the Roman Catholics, Anglicans, and Protestants. Which group Tallis agreed with seems to be hard to pin down, but Byrd was emphatically a Catholic (in his Mass for Three Voices, he has all of the voices normally doing their own thing come together for the word "catholicam" in the Credo and the again drifting apart). The view at the time was that musicians were no threat to the social harmony and so Byrd was tolerated.
"Infelix ego" is a meditation on Psalm 51 written by Girolamo Savonarola, a zealous and strict Dominican friar who became ruler of Florence from 1494-1498 before being excommunicated and deposed. He was tortured into signing confessions of heresy and other crimes; it was in remorse for this that he composed "Infelix ego," which means "Unhappy [wretch] [that] I am." This writing was one of the handful of Savonarola's works which escaped being put on the Index Librorum Prohibitorum and was set by many composers, especially in England. I couldn't find the Latin words online (here's the English translation), so I copied them down from the booklet accompanying another album with a different version (I'm going to go out on a limb and say that whatever claims to copyright may have ever existed are long gone by now).
Infelix ego, omnium auxilio destitutus, qui coelum terramque offendi. Quo ibo? Quo me vertam? Ad quem confugiam? Quis mei miserabitur? Ad coelum levare oculos non audeo, quia ei graviter peccavi. In terra refugium non invenio, quia ei scandalum fui.
Quid igitur faciam? Desperabo? Absit. Misericors est Deus, pius est salvator meus. Solus igitur Deus refugium meum: ipse non despiciet opus suum, non repellet imaginem suam.
Ad te igitur, piisime Deus, tristis ac moerens venio, quoniam tu solus spes mea, tu solus refugium meum. Quid autem dicam tibi? Cum oculos levare non audeo, verba doloris effundam, misericordiam tuam implorabo, et dicam: Miserere mei, Deus, secundum magnam misericordiam tuam.
Find and listen to the version by Oxford Camerata. I've heard the version by Stilo Antico, and while it's good, there's an important difference between the two. To me, Stilo Antico's version is a little more down-to-earth: it's slower, and the climax of the song is done less hauntingly. In the Oxford Camerata version, the climax occurs with "Miserere mei, Deus" at 11:42, but please trust me when I tell you not to go there immediately. That climax is one of the most beautiful moments in all of music; I had the album playing in the background as I was working and when it happened I was just frozen and transfixed by it. You need to listen to the piece as a whole to get the most of it, though.
I downloaded it from eMusic, but Napster and iTunes have it as well. Remember that it's the version by Jeremy Summerly & Oxford Camerata, and that it's by Byrd, not Lassus.