15 For if their rejection brought reconciliation to the world, what will their acceptance be but life from the dead? 16 If the part of the dough offered as firstfruits is holy, then the whole batch is holy; if the root is holy, so are the branches.
17 If some of the branches have been broken off, and you, though a wild olive shoot, have been grafted in among the others and now share in the nourishing sap from the olive root, 18 do not consider yourself to be superior to those other branches. If you do, consider this: You do not support the root, but the root supports you. 19 You will say then, “Branches were broken off so that I could be grafted in.” 20 Granted. But they were broken off because of unbelief, and you stand by faith. Do not be arrogant, but tremble. 21 For if God did not spare the natural branches, he will not spare you either.
22 Consider therefore the kindness and sternness of God: sternness to those who fell, but kindness to you, provided that you continue in his kindness. Otherwise, you also will be cut off. 23 And if they do not persist in unbelief, they will be grafted in, for God is able to graft them in again.
Yes, God did not spare the natural branches (the Jews) because of their unbelief in Christ (1 Thess 2:14-16), hence they were broken off. While most Jews rejected Christ many Gentiles accepted Christ therefore the Gentiles were graffed in. Tough God cast off the Jews God did not make salvation impossible for the Jews for if the Jews will not abide in unbelief (if they become believing Christians) they can be graffed in AGAIN.