ROMANS.11: = 19 You will say then, “Branches were broken off that I might be grafted in.” 20 Well said. Because of unbelief they were broken off, and you stand by faith. Do not be haughty, but fear. 21 For if God did not spare the natural branches, He may not spare you either. 22 Therefore consider the goodness and severity of God: on those who fell, severity; but toward you, goodness, if you continue in His goodness. Otherwise you also will be cut off. 23 And they also, if they do not continue in unbelief, will be grafted in, for God is able to graft them in again.
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The Jews were broken off from the natural olive tree because of unbelief = they rejected Christ.
....... Natural olive tree = God's tree of Judaism/Old Testament and New Testament
Gentile Christians have been grafted into the natural olive tree and they stand by faith (in Christ). If Gentile Christians do not continue in God's goodness, they too will be cut off because of unbelief or faithlessness or lost faith or renounced faith, and not because of sins/evil-works/no-works/no-fruits/disobedience.
....... It is unbelief or faithlessness which leads to sins/evil-works/no-works/no-fruits/disobedience.
P S - Gentile Christians are considered wild olive trees because most of them had previously led lawless lives from childhood, wrt Moses Law.