A good friend asked me a simple question that is good to always think through: How exactly is it that someone who believes in modalism (or anything other than the historic doctrine of the Trinity) is definitely not a Christian? In other words, how do we know for sure that the doctrine of the Trinity is a first-order, absolute essential to the Christian faith? How is anything but the Trinity a heresy?
Before I answer, modalism specifically is a heresy made most popular in the 3rd century by a man named Sabellius. It is the teaching that the Father, Son and Spirit are not three distinct Persons, but simply different modes of the same person, who reveals himself in those three ways at different times. And that heresy has showed up in slightly different forms ever since.
Also, my friend who asked the question already believed the Trinity to be essential; he simply was looking for clarity on how to articulate this answer. So that’s my hope here.
Here are two ways I would explain why you must believe in the doctrine of the Trinity or you will go to hell:
1. the easy answer: The doctrine of the Trinity is a summary of some of the clearest teachings in Scripture. As many great preachers have said, “Trinity” simply affirms seven undeniable truths: there is one God, the Father is God, the Son is God, the Spirit is God, the Father is not the Son, the Son is not the Spirit, and the Spirit is not the Father. If you deny the Trinity, you are denying at least one of those things, which simply means you refuse to listen to God’s clear revelation. If you do not believe the Trinitarian teachings of Scripture, you may as well chuck the whole Bible; you are not a Christian.
2. the more complex answer, but perhaps more convincing answer: to deny the Trinity is to deny the gospel. The gospel is the good news that the Father sent the Son into the world to save sinners for all eternity. The Son obeys the Father, by the Spirit’s power, in the place of sinners. The Father puts the Son to death. The Spirit raises the Son from the dead. The Son is exalted to the right hand of the Father, and the Father and the Son sent the Spirit to be the ultimate blessing to all believers. Without the Trinity, there is no gospel event. Without the Trinity, there is no salvation.
This is why we must pray for men like Benny Hinn and T.D. Jakes to repent and believe in Jesus Christ. Anyone who fudges at all on the Trinity simply does not understand the gospel. This is why you can be confident that there are zero Christians in the Unitarian church down the road from you, and why you must engage them with the gospel. “Believe in the Trinity” is another way of saying “repent and believe the gospel.”