In another article (192) we argued that religious unity could only exist around a central core of commonly held convictions, but we ran out of space before we could identify any of them. In this article we want to identify what is essential in order to have Christian unity.
The first core doctrine involves the teaching of trinitarianism. God the Father is God, but He is not God alone. Jesus Christ, the Son of God, is also God. The Holy Spirit of God is also God, but He is neither the Father, nor the Son. The New Testament teaching is that there is one God with a multiplicity of personalities within the Godhead. Unity with diversity. Without this doctrine, there can be no unity among religious people because those who believe in the deity of Christ cannot accept either the rejection of that deity, or the exaltation of that deity to the point where Jesus is considered the Father and the Spirit. Thus unitarian theologies (Judaism, Islam, or any of the varieties of unitarianism) are hopelessly conflicted with orthodox Christianity and there is no salvation for them in Christ.
The second core doctrine is the teaching that salvation is only in Jesus Christ. He Himself said that He was the only way to God and that no one came to the Father except through Him. There is no way that those who believe that can accept the dogma of so many today that salvation is in sincerity (it doesn't matter what you believe as long as you are sincere). There is such a thing as being sincerely wrong. Salvation is only in Christ. There can be no unity unless this core truth is commonly held. What fellowship has light with darkness?
The third core doctrine is the doctrine of salvation in Christ by grace alone through faith alone in Christ alone to the glory of God alone. Grace means God initiates for fallen man. Faith means man does not attempt to provide a behavioral basis for justification before God. Christ is the only object of faith. God is the initiator and ought, therefore, to receive all of the glory for our salvation. All who try to insinuate man and his behavior into the process deny all four of the parts of this doctrinal core, and cannot have salvation.
The fourth core doctrine is the teaching that disputes are only to be settled through the study of the Bible as the Word of God, communicated to men in the conventions of real languages. In other words, there can be no unity between people who will not study the Bible together in order to determine what the Truth really is. Those who hold to traditions, magisteriums, the teachings and conclusions of scholarly men, and set aside the study of the Scriptures as something they are unqualified to do cannot ever have unity with those who are committed to the Bible as the Word of God to every man. How can agreement come if those involved in dispute refuse to study together in the same Word with the same principles of interpretation?
This is the most basic core and unity cannot be without it.