Thomas Tallis is renowned for his wonderful choral pieces. Spem in Alium is a polyphonic motet written for 40 voices in an 85-part choir. We are not sure why Tallis decided to write this huge piece for 40 voices or even why he chose specifically 40 voices, but it is a beautifully splendid piece. Let's keep in mind that this was pop of his time and most of the music was written for church services, as most people in Europe at the time were professing Christians.
The words are from a church service and they talk about God's gracious character to deal with man's sins, since man cannot fix the wrongs that he has done. So this entire motet is a personal prayer asking God for forgiveness. In the Catholic church, their services were in Latin, since it was the predominant language for a while when all of Europe was a part of the Roman empire. Here they are translated in English :
I have never put my hope in any other but in You,
O God of Israel
who can show both anger
and who absolves all the sins of suffering man
Creator of Heaven and Earth
be mindful of our lowliness