I know that every doctrine in the Bible is important because all come from God. My question is, what doctrines must a person believe to be considered Christian? (For example, there will be Catholics in heaven who died trusting in Christ as their Savior, while there will not be Mormons in heaven since their doctrines involving Christ are not scriptural.) Is there a "key doctrine list" somewhere?
The apostle Paul’s answer to the jailer of Philippi’s question highlights a doctrine that is absolutely essential for salvation: “Believe in the Lord Jesus, and you will be saved” (Acts 16:31). Faith in Jesus Christ as Savior—justification through faith alone, by grace alone—is necessary for salvation. Saving Christian faith recognizes Jesus Christ as the Son of God, the second person of the Trinity. For this reason, the Athanasian Creed states that the doctrine of the Trinity and the doctrine of Jesus are essential for salvation.
As you indicated, all doctrines of the Bible are important. Because Christian doctrines are not isolated and stand-alone teachings, a denial of one doctrine has the potential of leading to the denial of more and more doctrines, and eventually threatening saving faith. This is why we seek to remain firmly grounded in God’s word always (Ephesians 3:14-19).