Jeremiah 1:17-19

How needful must faithfulness be in the service of the Lord? And where the Lord gives grace to his poor servants to be faithful, he will give strength to bear them up against all their foes. An iron pillar and brasen walls, are strong figures, to intimate, that the arrows of the enemy with all their fury will be thrown against them. But as iron pillars and brasen walls are simply for defense, and not for injury: how beautifully do those similitudes teach, that the servants of the Lord are not to strive and repel carnal attacks, with the same weapons, but to be like their Master, who when reviled, reviled not again. The Lord hath undertaken his servants cause, and he will recompense vengeance on his and their enemies, Deuteronomy 32:43.