1 Corinthians 4:21

By "rod" Paul meant the compelling power of the Spirit, which he had used against Elymas and which God had used against him..

The rod does not mean that there is no love, but love is hidden behind its blows and is not perceived by the one who is on the receiving end.

"What will ye? Shall I come unto you with a rod, or in love and a spirit of meekness?" There is much both of terror and of gentleness in this saying. For to say, "I will know," was the language of one as yet withholding himself: but to say, "What will ye? Must I come unto you with a rod?" are the words of one thenceforth ascending the teacher's seat, and from thence holding discourses with them and taking upon him all his authority. What means, "with a rod?" With punishment, with vengeance: that is, I will destroy; I will strike with blindness: the kind of thing which Peter did in the case of Sapphira, and himself in the case of Elymas the sorcerer. For henceforth he no longer speaks as bringing himself into a close comparison with the other teachers, but with authority. And in the second Epistle too he appears to say the same, when he writes, "Since ye seek a proof of Christ speaking in me." "Shall I come with a rod, or in love?" What then? to come with a rod, was it not an instance of love? Of love it was surely. But because through his great love he shrinks back in punishing, therefore he so expresses himself. Further; when he spoke about punishment, he said not, "in a spirit of meekness, but, [simply,] "with a rod:" and yet of that too the Spirit was author. For there is a spirit of meekness, and a spirit of severity. He doth not, however, choose so to call it, but from its milder aspect. And for a like reason also, God, although avenging Himself, has it often affirmed of Him that He is "gracious and long-suffering, and rich in mercy and pity:" but that He is apt to punish, once perhaps or twice, and sparingly, and that upon some urgent cause. Consider then the wisdom of Paul; holding the authority in his own hands, he leaves both his and that in the power of others, saying. […]

See the care, and wisdom, and power of the Instructor: "He shall not judge according to opinion, nor according to report; but He shall dispense judgment to the humble, and reprove the sinners of the earth." And by David: "The Lord instructing, hath instructed me, and not given me over to death." For to be chastised of the Lord, and instructed, is deliverance from death. And by the same prophet He says: "Thou shalt rule them with a rod of iron." Thus also the apostle, in the Epistle to the Corinthians, being moved, says, "What will ye? Shall I come unto you with a rod, or in love, in the spirit of meekness? " Also, "The Lord shall send the rod of strength out of Sion," He says by another prophet. And this same rod of instruction, "Thy rod and staff have comforted me," said some one else. Such is the power of the Instructor-sacred, soothing, saving.

He calls "rod" punishment. So, he says: it depends on you whether I come to you with the one or the other. If you remain careless, I will come and punish you, as Elymas (Acts 13:8–11). But if you sober up, I will treat you …