Core beliefs are those which we believe are essential to this interaction. We don’t change or compromise them. There’s an old saying attributed to Augustine, which says: “In essentials, unity; in non-essentials, liberty; and in all things, charity.”
What unites us as followers of Christ vastly outweighs what might divide us. In the essentials, such as the deity of Christ, the Trinity, justification by grace alone, through faith alone, because of Christ alone, etc., we need to be in agreement. We cannot compromise on these major issues of the Gospel. These are “non-negotiables”; absolutes, if you will. In fact, to depart from these doctrines is to depart from the Christian faith itself.
But anything else is up for grabs. Christians hold differing views about important, yet non-essential matters. For instance, whether someone believes in the baptism of infants or whether God still heals today, are important issues; yet these are not essential to someone being included or excluded from the kingdom of God. Someone is not a “false teacher” who takes a different position on these issues.
The same is true for doctrines such as whether someone believes in a “pre-tribulation” rapture or a “post-tribulation” rapture or for those who take different positions on the millennial reign of Jesus Christ. Sincere, godly, dedicated believers believe different things about these issues, but it does not mean that one person is saved and another damned because they have a different view.
Scripture commands us to “maintain the unity of the Spirit” (Ephesians, chapter 4, verse 3) “until we all come to the unity of the faith.” (Ephesians chapter 4, verse 13).