Romans 11:21-23

²¹For, if God did not have mercy on the natural branches, he will not have mercy on you.

²²See then that God is good but his rules are fixed: to those who were put away he was hard, but to you he has been good, on the condition that you keep in his mercy; if not, you will be cut off as they were.

²³And they, if they do not go on without faith, will be united to the tree again, because God is able to put them in again.