Which is why I am urging my brother to be very, very careful.I believe it was in his letter to the Romans, Paul noted that God's law/way is written on the hearts of all. He noted that non-believers would be judged by their own hearts.
On Pentecost, the Holy Spirit descended on Church, and has been present since. My belief is that Jesus, through the Holy Spirit, reaches all at the place where they are. Remember how Jesus went out to people. Meddling with the churches, faith, beliefs of others, is meddling with the Holy Spirit, spitting on the Spirit's work.
Telling others what is wrong with them/their faith/their religion means a loss of opportunity to share how faith has worked/is working in one's own life.
We should all be able to note the ways God and his Holy Spirit is working in different faiths, different religions, different churches. This is his world, and he does make his presence known to those who have eyes that are not blind.