# This is why he also says in another Psalm

"In another Psalm of David, he also says about the Messiah" 

# he also says

"David also says." David is the author of Psalm 16, from which this quotation is taken.

# You will not allow your Holy One to see decay

"You will not allow the body of your Holy One to rot" 

# You will not allow

David is speaking to God here.