God has revealed much about Himself to humans. His dealings with humans, and especially with the descendants of Abraham, are chronicled in the Old Testament. The words and life of Jesus and his followers are chronicled in the New Testament.
Thus the Bible becomes the chief revelation of God to humanity. When these revelations are ordered into principles and categories of truth we begin to create doctrine.
When doctrine is substituted for faith then we have a form of propositional Christianity. That is, our life in Christ is measured by what we believe. The propositions become the mark of the believer.
The advantage to propositional Christianity is that it protects us from heresy. The disadvantage is that is overlooks the heart of the believer. That is, a thought based (mental) form of Christianity often results where Christian character, behavior, attitudes and spirit are neglected and minimized.
The revelation of God itself contains the truth that He gives us His Spirit to live inside of us. Thus the indwelling spirit is part of God’s truth, and our walk with God must go beyond our mind and our beliefs. To be Christian is to have a new heart. To know God results in being transformed and not merely educated.
When we become children of God we enter into a relationship with Him. This relationship forms the basis of relational Christianity.