thou standest by faith — there is no personal merit that earned Gentiles their opportunity — faith excludes boasting (Romans 3:27)
fear — Gentiles should fear stumbling as Israel did.
Incalculable harm has been done by those who have attempted to apply our text primarily to individuals. An individual who is once grafted into Christ by faith can never be broken off from Christ. He is not going to be a mutilated trunk in Heaven forever … When it is seen that Paul is speaking of the Jews’ being removed nationally, to be replaced nationally by Gentiles, we are fully prepared for that which will follow in a few verses. — Barnhouse, page 144.