Jamieson, Fausset and Brown on Romans 12:18

18. If it be possible--that is, If others will let you.

as much as lieth in you--or, "dependeth on you."

live peaceably--or, "be at peace."

with all men--The impossibility of this in some cases is hinted at, to keep up the hearts of those who, having done their best unsuccessfully to live in peace, might be tempted to think the failure was necessarily owing to themselves. But how emphatically expressed is the injunction to let nothing on our part prevent it! Would that Christians were guiltless in this respect!