Accepting the declaration of any human being except God in relation to what is lawful or unlawful means taking that human being as the Lord!

Verse 31 of Chapter 9

They (Jews and Christians) took their rabbis and their monks to be their lords besides Allah, and (they also took as their Lord) the Messiah, son of Maryam, while they were commanded to worship none but One God, none has the right to be worshipped but He. Praise and hallowed be He above what they associate (with Him).