11 Such and so manifest are the proofs of our Saviour's power and energy after death. Who then can be found of spirit so obdurate as to withhold his assent to the truth, and refuse to acknowledge his life to be Divine? Such deeds as I have described are done by the living, not the dead; and visible acts are to us as evidence of those which we cannot see. It is as it were an event of yesterday that an impious and godless race disturbed and confounded the peace of human society, and possessed mighty power. But these, as soon as life departed, lay prostrate on the earth, worthless as dung, breathless, motionless, bereft of speech, and have left neither fame nor memorial behind. For such is the condition of the dead; and he who no longer lives is nothing: and how can he who is nothing be capable of any act? But how shall his existence be called in question, whose active power and energy are greater than in those who are still alive? And though he be invisible to the natural eye, yet the discerning faculty is not in outward sense. We do not comprehend the rules of art, or the theories of science, by bodily sensation; nor has any eye yet discerned the mind of man. Far less, then, the power of God: and in such cases our judgment is formed from apparent results.
12. Even thus are we bound to judge of our Saviour's invisible power, and decide by its manifest effects whether we shall acknowledge the mighty operations which he is even now carrying on to be the works of a living agent; or whether they shall be ascribed to one who has no existence; or, lastly, whether the inquiry be not absurd and inconsistent in itself. For with what reason can we assert the existence of one who is not? Since all allow that that which has no existence is devoid of that power, and energy, and action, for these are characteristics of the living, but the contrary is characteristic of the dead.
Source: Oration in Praise of Constantine (New Advent)