It's plainsong, in this case specifically a Gregorian chant on the motet "Tu es Petrus" (from the Biblical text "You are Peter, and upon this rock I build my church"). Plainsong dates back to the early Christian church in Roman times, and Gregorian chant from the 6th century AD. It was originally written without lines, and then with four lines to a staff, which is still most common. Five lines originated in the 13th century, and as this has five lines it must be from some time after then. How long after then? Hard to say. Gregorian chant has been written and more recently printed from ancient times right into the 21st century. It obviously looks hand written rather than printed, and bears stylistic features (such as the colouring and style of the first letters of each sentence) which make the 1600s a plausible possibility, but I'm very far from an expert so I honestly couldn't say with any certainty at all when it was from. A musical antiques specialist should be able to date it without too much trouble, if you showed it to them.
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