A bruised reed he will not break, a smoldering wick he will not quench, until he brings justice to victory.
We might think of the bruised reeds and smoldering wicks as those opposed to Jesus:
St. Augustine: So He neither bruised nor quenched the Jewish persecutors, who are here likened to a bruised reed which has lost its wholeness, and to a smoking flax which has lost its flame; but He spared them because He was not come to judge them, but to be judged by them.
St. John Chrysostom: The Lord sought to heal the Jews by this mildness. But though they rejected Him, yet He did not resist them by destroying them.... "He shall not break a bruised reed," shews that it was as easy for Him to break them all, as to break a reed, and that a bruised reed. And, "He shall not quench a smoking flax," shews that their rage was fired, and that the power of Christ was strong to quench such rage with all readiness; hence in this is shewn the great mercy of Christ.
Of course, sets of two often remind the Fathers of the Jews and the Gentiles. St. Hilary matches them this way:
He means this bruised reed that is not broken, to shew that the perishing and bruised bodies of the Gentiles, are not to be broken, but are rather reserved for salvation. "He shall not quench a smoking flax," shews the feebleness of that spark which though not quenched, only moulders in the flax, and that among the remnants of that ancient grace, the Spirit is yet not quite taken away from Israel, but power still remains to them of resuming the whole flame thereof in a day of penitence.
St. Jerome proposes it the other way around:
He calls the Jews a bruised reed, whom tossed by the wind and shaken from one another, the Lord did not immediately condemn, but patiently endured; and the smoking flax He calls the people gathered out of the Gentiles, who, having extinguished the light of the natural law, were involved in the wandering mazes of thick darkness of smoke, bitter and hurtful to the eyes; this He not only did not extinguish, by reducing them to ashes, but on the contrary from a small spark and one almost dead He raised a mighty flame.
St. Jerome also proposes a more generic application, which may be the most common today:
He that holds not out his hand to a sinner, nor bears his brother’s burden, he breaks a bruised reed; and he who despises a weak spark of faith in a little one, he benches a smoking flax.
All these interpretations, of course, preserve the basic point of something feeble that ought to be strong, yet toward which God is merciful until the end.
I'll add, though, that reeds and wicks are not inherently images of strength and endurance. If we ourselves are not bruised or smoldering, we might still regard ourselves as not more secure against violence, absent Christ's mercy, than even whole reeds or brightly burning wicks.