# This is he of whom it was written

 "He is the one about whom a prophet wrote long ago" or "This is what a prophet wrote long ago about John the Baptist"

# I am sending my messenger

The pronouns "I" and "my" refer to God. The prophet was quoting what God said.

# before your face

"in front of you" or "to go ahead of you"

# prepare your way before you

This means the messenger will prepare the people to receive the Messiah's message.