11:21a

For: This word introduces a reason that a believer should fear.

if: Here the word if introduces a situation that is true. If this first part is true, that supports the second part (God will not spare the readers if they no longer believe in Jesus (11:20b)) to also be true.

In some languages, translating literally here would have a different meaning. If that is true in your language, translate the correct meaning. For example:

if ⌊it is true that⌋ God did not spare the natural branches
-or-
God did not spare the natural branches, so he will not spare you, ⌊if you stop believing in him⌋

spare: The word spare refers to preventing harm or trouble coming to someone. Here this phrase indicates that God did not protect the Jews who did not believe in Jesus from the consequences of that unbelief. Here are other ways to translate this phrase:

did not hold back ⌊from harm⌋
-or-
did not protect

See how you translated this word in 8:32.

the natural branches: This refers to the same branches as in 11:19. These branches are natural because they belonged to the tree. They grew from the tree. They are not the grafted-in branches. Here are other ways to translate this phrase:

the original branches (New Living Translation (2004))
-or-
the true branches
-or-
the branches belonging to the tree

11:21b

He will certainly not spare you: This clause implies the same condition as the natural branches. If the readers of this letter become unbelieving like some Jews were, God will not spare these readers either. In some languages all or some of that information needs to be explicit to make the translation clear. For example:

if you do not believe in Christ he will part from you
-or-
he certainly will break you off too if you abandon (lit. drop) your belief-in/obeying-of him

either: Here this word means “similarly not.” In the same way that God did not spare the Jews who did not believe, he will also not spare Gentile believers if they stop believing.

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