20 The Greek construction here implies that such suffering was not the norm, even though it could happen, and in fact may well have happened to some of the readers (cf. 4:4, 12–19).
21 Grk “because of righteousness.”
22 Grk “do not fear their fear,” referring to those who cause their suffering. The phrase “their fear” may mean “what they fear” (subjective genitive), but in a situation of persecution it more likely means “fear of them” (objective genitive).
23 A quotation from Isa 8:12.

In other versions

But even if you should suffer for righteousness’ sake, you will be blessed. Have no fear of them, nor be troubled,

But even if you should suffer for what is right, you are blessed. “Do not fear their threats; do not be frightened.”

But even if you should suffer for righteousness, you are blessed. Do not fear them or be intimidated,

But even if you suffer for doing what is right, God will reward you for it. So don’t worry or be afraid of their threats.

But even if you might suffer for the sake of righteousness, you are blessed. And do not be afraid of their intimidation or be disturbed,

But and if ye suffer for righteousness’ sake, happy are ye: and be not afraid of their terror, neither be troubled;

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