Christ must reign. The ‘must’ is very emphatic in the original Greek.
All the victories that Christ gains will be given to his people. They too will trample Satan under their feet, and to his eternal shame the weak will prevail over the strong (Romans 16:20). The saints will receive immortality and like Christ will rise to die no more, for death will no longer have power over them (Romans 6:9-11). They will put on immortality (1 Corinthians 15:53-54; 2 Timothy 1:10).
The resurrection displays in a unique way Christ's absolute irresistible power to keep his promises, and particularly his great promise to resurrect every one of his people at the last day. ‘He must reign until finally he has put all enemies under his feet.’ Christ was dead and buried, and by the exertion of his mighty power, he has broken the bands of death, and done the greatest possible thing from the human standpoint, and defeated death itself death. When you consider that you realize that he must reign. What can stop him reigning? It would be wrong to say that at some point in the future he will begin to reign. That would suggest that there is some influence more powerful than he is, until some future time comes. No, says the apostle, the one who could be truly dead and buried, and could break the bands of death – how could such a person ever be ruled by anything? That is the ultimate demonstration of power and authority. If he has done that, nothing can possibly be ruling over him. And so while at the present time it may not always appear as though he is reigning, in reality he must be. In warfare, how often it happens that there is a mighty conflict and then there is some pivotal key battle towards the end of the war, which really seals everything. After that the enemy has no spirit. His lines of communication are broken; his central organization has lost the majority of his forces. And yet the war may drag on for maybe another six months or so. Yes, but from that pivotal battle it was all a foregone conclusion. It was really, as they say, just the mopping up operation, just dealing with the last pockets of resistance. That is what the apostle is saying. Calvary and the resurrection: those are the great pivotal battles – the bearing away of sin, the breaking of the bands of death. After those he is reigning. It doesn't matter what may appear.