To give his cheek to the one who strikes
is an expression for passively enduring abuse. Such striking was an act of utter disrespect, but it is to be endured because it was purposed by God himself as part of his discipline (see Isaiah 50:6).1 For this reason, even if it is people who insult and abuse someone, one should not retaliate, but learn to bear such violence patiently, with trust in God. Accompanying this is the further instruction to be filled with insults.
It means to accept insults in meekness, in the sense of suffering full reproach (see Job 10:15; Psalm 88:4; Psalm 123:3). Here it is not to be understood purely passively; rather the meaning is to allow oneself to be satiated with reproach.2 Because these afflictions were purposed by God for a reason, it should be accepted and endured as positive elements of faith.3
30 let him give his cheek to the one who strikes, and let him be filled with insults.