Romans 11 is clear,we can only be grafted in by faith. Salvation was offered to the gentiles because of the disbelief of many of the nation of Israel. It is offered, we do not automaticaly become Christians, we have to make a choice.  Those who were removed were removed for disbelief. Can a believer change his mind and turn away from God, sure. If it were not so we would never have been warned to make sure of our calling, we would never have been told to be not part of the world, etc etc. How did some or many of the Jews removed, by disbelief. That is how anyone can be removed, it is not just limited to the Jews.
 
 
Romans 11:18
 
do not boast over those branches. If you do, consider this: You do not support the root, but the root supports you. 19You will say then, "Branches were broken off so that I could be grafted in." 20Granted. But they were broken off because of unbelief, and you stand by faith
 
Jesus will never leave you are forsake you. What about you. Will you leave him? Because he loves us he will never force us to make a decision even if it is terribly wrong.