Some of them are maybe true Christians inspite of their church's teachings. If the Holy Spirit does a thorough work in them they'll want to come out.
We are commanded to come out of apostate churches in which an antichrist/the Antichrist has been set up in:-
And I heard another voice from heaven, saying, Come out of her, my people, that ye be not partakers of her sins, and that ye receive not of her plagues. (Rev. 18:4)
People may think that they are strong enough Christians but it is more likely they will be influenced for bad than that they will influence others for good. I've seen this in the Church of Scotland where the antichrist of Liberal Theology has been set up for decades.
All Roman Catholics are nominally Christians, because they partake of a perverted version of baptism and the Lord's Supper, but that just brings them under greater condemnation, as it does with nominal Liberal Christians or others. The inner reality of the covenant must be there for salvation as well as an outward bond.
The percentage of Christians (Romanist, Eastern Orthodox, Protestant, Pentecostal) that are true Christians must be quite small (?) We need to be praying for God to do a great work on the Earth.
Knox Free Church,
Perth, Scotland GB
His Name forever shall endure;
last like the sun it shall:
Men shall be blessed in Him,
and blessed all nations shall Him call (Ps. 72:17)