From our ongoing collection: Golden Age Classics for SAB Choir. Smaller choirs DESERVE to sing these! SAB a cappella with optional accompaniment.
This motet was included by Morley in his treatise A Plain and Easy Introduction to Practical Music (1597) as an example of his principles of composition. It is suitable for Lenten or general use, but it is not liturgically suitable for the Ordinary of the Mass, especially since the text is not the same as that of the Mass. However, while the word nostri is substituted for the usual nobis, this is not grammatically incorrect by any means; the Latin verb misereor (I have mercy) takes the genitive case (here, nostri, another example being Miserere mei; nobis and mihi are the dative of these words, respectively), but often governs the dative in ecclesiastical Latin. (Just for your information!)
On the other hand, if you want to change the word to "nobis," we're sure Mr. Morley won't mind.
Agnus Dei, qui tollis peccata mundi, miserere nostri.
Lamb of God, who take away the sins of the world, have mercy on us.