The foundation of our Christian faith is Jesus Christ the son of God. He is the author & finisher of our faith :
looking unto Jesus the author and finisher of our faith; who for the joy that was set before him endured the cross, despising the shame, and is sat down at the right hand of the throne of God. Hebrews 12:2
There are foundational doctrines in the Word of God that one must believe in as a Christian. You will find them in most Statements of Faith of Christian churches or Christian web pages. The Bible states:
” Whosoever transgresseth (G3845 = depart, leave, be turned from), and abideth (G3306 = remain, stay, continue) not in the doctrine of Christ, hath not God. He that abideth in the doctrine of Christ, he hath both the Father and the Son. If there come any unto you, and bring not this doctrine, receive him not into your house, neither bid him God speed: For he that biddeth him God speed is partaker of his evil deeds.” 2 John 1:9-11 .
The ‘Doctrine of Christ’ –doctrine means teachings, instructions. For this reason many consider the foundational doctrines to be all the teachings & instructions of Jesus, found in the New Testament. The book of Hebrews points out six foundational teachings (called doctrine of Christ) that every Christian should know & are considered milk. See Hebrews 6:1-2. However some Christians only consider the ‘Doctrine of Christ’ to be the teachings about the person, nature & works of Christ (Christology), which includes the Gospel of Salvation.