For if God did not spare the natural branches, neither will he spare you.  Note then the kindness and the severity of God: severity toward those who have fallen, but God's kindness to you, provided you continue in his kindness. Otherwise you too will be cut off. (ESV)
There is an old expression in the church that says, "God has no grandchildren." Some have made the mistake that they believe they are a Christian because their family is Christian. Their parents were Christian. Their nation is a Christian nation. There were many Jews who made that mistake. They were part of the nation of Israel, God's chosen nation. They did not follow God, or even know him.
God did not spare the Jews who did not belong to him. Even though they belonged to the Jewish nation by birth they did not follow his Law. They did not love him. They followed after other gods, worshiping idols and living as rebels to the true God who called them. These natural branches were cut off. God did not spare them.
Paul reminds the Gentiles that they should not make the same mistake that many of the Jews made. If the Gentiles continue in rebellion to God and never surrender their lives to the Gospel of Jesus Christ they will be like those branches that were broken off of the tree. God is loving. He is also just. He will not be mocked.
God's kindness will be shown to those who continue in his kindness. This is a wonderful point that we need to grasp. We are not at all saved by works. We have already established in Scripture that salvation is by grace. However, a truly saved person will demonstrate that they are continuing in the kindness of God by the works they are doing, which they were created to do.
Are you truly grafted into the family tree of God's kingdom? One will know by the works of righteousness which you are doing. You are not God's grandchild. You are either a child of God or a child of the devil.