"There is one God in three persons, the Father, the Son and the Holy Spirit." (For example, UCCF's Doctrinal Basis of Fellowship, point 1)There's a lot more to being a Christian but this is surely fairly key. Strange thing is that this is often considered fairly incidental rather than central and defining. CS Lewis famously called "grace" the defining mark, but grace is really an implication of being Trinitarian isn't it?
Ron Frost reflects on the marks of being truly Trinitarian.
1. Starting point, Trinity is key rather than just a secondary point about God. That means Trinity is on page 1 of your Systematic Theology rather than page 226 (as in Grudem's Systematic Theology.....). That God is Triune isn't the wierd mystery we leave for the theology freaks it's who God is, and turns out he is able to reveal himself.7. What else?
2. Christology, Christ is key. The centre of the Father's revelation and relationship with us is in the Son who is our bridegroom and the one in whom we're adopted.
3. Pneumatology, the Spirit is personal not a force, and is the person who brings us into the fugue of Triune life.
4. Love of God, is to be experienced.
5. Sin, "is a relational violation rooted in absolute disaffection (“hatred”) that Jesus overcomes by his own loving atonement."
6. Revelation "is no longer seen as merely contractual and rational but as the passionate and compelling disclosures of the Triune God’s love for us."