In what sense is religion the ace up Satan’s sleeve? If Christianity is not a religion, what is it?

Satan is the father of all religions, and religion is his ace because it wears the appearance of devotion while depending on man rather than God. The word religion in the New Testament always has to do with externals and ceremony, yet no amount of care in the externals of divine service can ever make a person acceptable to God. Satan happily fosters an infinite variety of religions, but they all have one thing in common: they rest on what man does. Christianity is not a religion at all; it is a relationship with the Lord Jesus Christ, a total grace operation in which entry, growth, and production all rest solely on the work of Christ on the cross received by faith, so that God does the work, God gets the credit, and man gets the blessing.