John Chrysostom, on 1Cor 9:26

"I therefore so run, as not uncertainly."

Thus having shamed them from those that are without, he next brings forward himself also, which kind of thing is a most excellent method of teaching: and accordingly we find him every where doing so.

But what is, "not uncertainly?" "Looking to some mark," saith he, "not at random and in vain, as ye do. For what profit have ye of entering into idol-temples, and exhibiting for-sooth that perfectness? None. But not such am I, but all things whatsoever I do, I do for the salvation of my neighbor. Whether I show forth perfectness, it is for their sake; or condescension, for their sake again: whether I surpass Peter in declining to receive [compensation], it is that they may not be offended; or descend lower than all, being circumcised and shaving my head, it is that they may not be subverted. This is, "not uncertainly." But thou, why dost thou eat in idol-temples, tell me? Nay, thou canst not assign any reasonable cause. For "meat commendeth thee not to God; neither if thou eat art thou the better, nor if thou eat not art thou the worse." Plainly then thou runnest at random: for this is, "uncertainly."

"So fight I, as not beating the air." This he saith, again intimating that he acted not at random nor in vain. "For I have one at whom I may strike, i.e., the devil. But thou dost not strike him, but simply throwest away thy strength."

Now so far then, altogether bearing with them, he thus speaks. For since he had dealt somewhat vehemently with them in the preceding part, he now on the contrary keeps back his rebuke, reserving for the end of the discourse the deep wound of all. Since here he says that they act at random and in vain; but afterwards signifies that it is at the risk of no less than utter ruin to their own soul, and that even apart from all injury to their brethren, neither are they themselves guiltless in daring so to act.