I think this is different than legalism. Revelation, by the way, does not directly name Roman Catholicism. And, technically, Catholics are not known for selling slaves, and slaves are mentioned in Revelation as one of the things of commerce in Babylon . . . right? But he might have an explanation for this.
I do understand how a number of Protestants loved a lot of the Catholic things, but they did not want to be ruled by a pope. You can still see some similarity between Catholic communion service and how certain Protestant groups have communion and make it the center of their worship.
Satan has counterfeits. This does not mean there is not the real thing. Counterfeit money is not evidence that there is no real money.
"As each one has received a gift, minister it to one another, as good stewards of the manifold grace of God." (
1 Peter 4:10)
As I get this, any real gift of God ministers God's own grace to change us into how Jesus is and how He has us pleasing God while loving as His family and ministering this to help to save lost people so they join us in this sharing as family with God. So, yes Satan's counterfeits do not minister this, plus some amount of church culture does not so minister God's own grace. But each of us in Jesus is able to minister God's own grace, I would say, through gifts and our prayer and good example.
And as we grow in Jesus, we grow in all His ability of God's own character of love.
So, if your friend does not minister this to you - - minister this to him, by prayer and example and whatever you can say to encourage the person.
Paul our Apostle uses "I" and "me", doesn't he?
You feed on God's word and keep changing, and be his good example of how to be gentle and humble and creative instead of arguing. And talk with him about how Jesus gives us rest for our souls (
Matthew 11:28-30); and offer how we are commanded to be ruled by God's peace in our hearts >
Colossians 3:15 > so we are personally ruled all the time by God's own peace keeping us with Him and making us creative for how to love each and every person while sharing and caring as His family..
And you yourself share with church people who help you with this, so you can learn in real life how to grow and relate as family in God's love.