Ephesians 4:4-6 4 There is one body and one Spirit, just as also you were called in one hope of your calling; 5 one Lord, one faith, one baptism, 6 one God and Father of all who is over all and through all and in all.
If a person is saved, a child of God, they are of the Christian faith. That a subscriber to either the WCF of 1689 LBC is bound to be wrong on an issue does not mean they are not holding the Christian faith and life.