This is a piece from the 16th century by Thomas Tallis called "If ye love me".
I started by coloring the sentences in the different voices to make it more visible to me. I think this was a good way, but then I started analysing the chords. This worked out for the first line (in red, at the top | first picture), but when the voices unsynchronize, it becomes a total mess (2nd picture). Any other ideas on how to analyze this?