28 Grk “and.” The Greek conjunction καί (kai) introduces the apodosis of the conditional sentence.
29 This is a first-class conditional sentence with εἰ (ei) + aorist indicative in the protasis. Reality is assumed for the sake of argument with a first-class condition. The author here assumes the reality of the protasis (the “if” clause), which his recipients, as believers, would also be expected to agree with: Assuming that God has loved us in this way, then it follows that we also ought to love one another. God’s act of love in sending his Son into the world to be the atoning sacrifice for our sins (v. 10) ought to motivate us as believers to love one another in a similar sacrificial fashion. The author made the same point already in 1 John 3:16. But this failure to show love for fellow believers is just what the opponents are doing: In 1 John 3:17 the author charged them with refusing to love their brothers by withholding needed material assistance. By their failure to love the brothers sacrificially according to the example Jesus set for believers, the opponents have demonstrated again the falsity of their claims to love God and know God (see 1 John 2:9).