On of my best friends is a Oneness Pentecostal and I’ve debated him for over 10 years since he became a Modalist. He was a non-denominational Christian before joining this Men’s outreach program which was put on by a Oneness Pentecostal church. For several months he was in this program and during that time he was taught that the Trinity was a man-made doctrine invented by the Catholic Church and unless one is baptized in Jesus name only and can speak in tongues, they aren’t validly baptized and will not enter God’s Kingdom.
I wish there was an easy “go to” argument to use against those who believe in Modalsim, but unfortunately there isn’t. I’d rank Oneness Pentecostals right up there with Jehovah’s Witnesses, in terms of being completely immersed in their beliefs and doctrines to the point that it almost becomes indoctrination. Another sad part is that at times, those Oneness Pentecostals that I’ve talked to, my friend included, can come across as very condescending and arrogant, when they are talking about other religions and trying to share their own faith. I’m not trying to generalize all Oneness believers that way, I’m merely sharing my own experiences.
The one area I’ve had the most success with is Sola Scriptura. To this day my friend has yet to give a reasonable answer as to why his church’s interpretation of scripture is correct and others are not. Plus the lack of historical evidence showing that the early church held to the beliefs of modern day Oneness Pentecostals has also been difficult for him to explain.
I don’t know what their official teaching is on the matter of there being only one true faith, but my friend is convinced that their is no “single Christian Church” that is the one true church, as Catholics believe. He feels that all churches have some truth, but yet he can’t explain how to prove that statement as being true without a valid way to prove the “true” teachings from the “false” ones.
I think many non-Catholics who think this way simply choose to ignore this dilemma and take comfort in knowing they are right, without actually having to prove it beyond a reasonable doubt.