And let not Celsus be angry if we describe as lame and mutilated in soul those who run to the temples as to places having a real sacredness and who cannot see that no mere mechanical work of man can be truly sacred. Those whose piety is grounded on the teaching of Jesus also run until they come to the end of their course, when they can say in all truth and confidence: “I have fought a good fight, I have finished my course, I have kept the faith; henceforth there is laid up for me a crown of righteousness.” And each of us runs “not as uncertain,” and he so fights with evil “not as one beating the air,” but as against those who are subject to “the prince of the power of the air, the spirit that now works in the children of disobedience.” Celsus may indeed say of us that we “live with the body which is a dead thing;” but we have learned, “If you live after the flesh, you shall die; but if you by the Spirit do mortify the deeds of the body, you shall live;” and, “If we live in the Spirit, let us also walk in the Spirit.” Would that we might convince him by our actions that he did us wrong, when he said that we “live with the body which is dead!”
Source: Against Celsus (New Advent)