Having recently completed a book, Your Church Is Too Small, I have given a great deal of thought to the question: "What must a Christian believe?" The answer to this question is the very basis upon which we can experience the unity of the Spirit in the bonds of peace with other Christians, even other Christians who believe much more, or much less, than we confess.
It seems that the most common objection to relational unity with other Christians is based upon a prior question, "What must we believe in order to enter into a relationship in Christ that does not compromise the historic Christian faith in any essential way?" Doctrine matters because Christianity is more than a relationship. It is a relationship, and it includes a vital experience that comes in being united with the living Christ by the Holy Spirit. But this relationship and experience is rooted in teaching, or doctrine. If you do not have a doctrinal basis for this relationship then you may have the wrong relationship, or even a dangerous relationship. This truth is routinely abused by all types of fundamentalist Christians, from all three of the great traditions, but it is a relationship that we must insist upon if we are to be faithful to Jesus Christ.
The sectarian answer to this question will always insist on an approach that says, in effect: "My (our) way or the highway." The sectarian will take a whole system of doctrinal affirmations, including a number of them that are not common to the great Christian tradition, and make these the test of all relationships.
The latitudinarian answer to this question will say: "Believe whatever you will so long as we love one another and remain tolerant." Christians from many traditions, and supposedly from no tradition, refuse to have anything to do with a doctrinal basis for relationship, insisting that doctrine divides and love unites. The core problem here is that these Christians appeal to the love of Christ without affirming faith in the person of Christ. This approach will eventually destroy vital Christianity by reducing it to an ethical framework without a biblical and historical basis.
The Heidelberg Catechism, in Question 22, asks: "What then must a Christian believe?"
Everything God promises us in the gospel (cf. Matthew 28:18-20; John 20:30-31). That gospel is summarized for us in the articles of our Christian faith—a creed beyond doubt and confessed throughout the world.
What follows, in questions 23-58, are careful and biblical applications of the articles of the Apostles' Creed.
I would argue that the framers of this catechism got it right. What every Christian must believe is to be found in the Apostles' Creed. Roman Catholics believe much more than what is in this creed. So do the Orthodox and all Protestants. All who are faithful to the core orthodoxy of Christianity and the essential teaching of the Bible believe in their heart and soul what this creed teaches. This is so because the creed teaches, in a simple summary fashion, what the Bible teaches. Again, there is more taught in the Bible than is taught in this short creed but what is taught in this creed "must" be believed by all Christians.
This approach does several important things. First, it promotes core orthodoxy and properly refuses Christian standing to cults and non-Christian heresies. Second, it keeps the faithful centered on what is primary and essential to real faith. Third, it reminds us that all Christians, regardless of their earthly communion and various beliefs, share in this "mere Christianity."
Everything I teach and everything I practice is passed through this holy test. Is the faith being confessed in agreement with the faith confessed by the earliest Christians, those who came right after the apostles and prophets of the New Testament? The Apostles' Creed has the unique role of history and place and unites us where we can and must be united.
This is why I have such precious unity of the Spirit with so many Christians with whom I disagree on one or more points of faith and/or practice. The unity I share is rooted in the experience we have in Christ and the faith that we share in the teachings of the earliest Christians. I long to see a growing army of Christians adopt this simple, but not simplistic, approach to unity. If we are baptized followers of Jesus Christ, who confess explicit faith in the Lord of heaven and earth, and affirm that our oneness is found in confessing together what the church has confessed from the earliest times then we can share a deep and growing relational unity with one another. We may not share that relationship in the same church communion on earth but we can share it in common human relationships since we are truly brothers and sisters in Jesus Christ. We will all share this faith in the church triumphant so it only makes sense to find ways that we can share it now in the church militant. This is my ecumenism.