When Christians begin to think about and discuss unity, the question of doctrine always comes up. How do we overcome doctrinal disagreement?
Augustine, a Christian brother who lived in the fourth century, answered that question this way: “In essentials, unity; in non-essentials, liberty; in all things, love.”
Every belief system (and/or worldview) has essential beliefs. For instance, it is essential to atheism to believe that there is no such thing as God, a god, or gods. If someone believes in God, by definition, she/he is not an atheist.
Every belief system also has non-essential beliefs. For instance, one atheist may believe in the Big Bang and another atheist may believe that the universe has always existed. However, they are both still atheists because neither of those beliefs is essential to atheism.
So what are the essential truths of Christianity? The book of 1 John, tells us how to recognize them. "Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, but every spirit that does not acknowledge Jesus is not from God. This is the spirit of the antichrist."
According to John, if a spirit (or doctrine) acknowledges that Jesus has come in the flesh it is of God. If it doesn't, it is not from God. So certain beliefs about who Jesus is, are essential. If we redefine Jesus and deny who He is, we have broken with essential Christian teaching.
So how do we know who Jesus is? Fortunately, Jesus told us who He is, and we have His words in Scripture. He told His disciples and we also have many of their words. Jesus also revealed Himself through Old Testament writers and we have their words as well.
Jesus is God in human flesh. He is the Messiah (the Christ) that the Jews were looking for. He is the sacrifice for our sin. He is Lord (Absolute Master). He is bodily risen from the dead. He is present as the Spirit when we gather in His name. He is "the same yesterday, today, and forever."
If someone says they are a Christian, yet continually affirms a doctrine that denies or redefines who Jesus is, they have broken with Christianity. Paul confirms this by warning us against following "another Jesus" (a redefined Jesus).
As John wrote, some teachings (and the spirits behind those teachings) are not from God. Both Jesus and many of His first followers warned us to watch out for false prophets. We need discernment and wisdom not to follow or support any teachings or teachers that redefine who Jesus is.
So in order to have Christian unity we unite around Jesus. It is important that we unite on the essentials of who He is. This doesn't mean that we try to force or coerce people to believe the essentials, but that we don't try to have Christian unity with people who redefine Jesus. We need to love them and serve them, but we shouldn't accept their doctrines that redefine Jesus, to be Christian teachings.
As far as other doctrines that don't redefine Jesus, Christ-followers have much liberty and diversity in our beliefs. We all see in part so let's love one another (and all people) in spite of our doctrinal differences, "speaking the truth in love."