2 Yet I exhort you by our common faith, by the true and simple love of my heart towards you, that, having overcome the adversary in this first encounter, you should hold fast your glory with a brave and persevering virtue. We are still in the world; we are still placed in the battlefield; we fight daily for our lives. Care must be taken, that after such beginnings as these there should also come an increase, and that what you have begun to be with such a blessed commencement should be consummated in you.
It is a slight thing to have been able to attain anything; it is more to be able to keep what you have attained; even as faith itself and saving birth makes alive, not by being received, but by being preserved. Nor is it actually the attainment, but the perfecting, that keeps a man for God. The Lord taught this in His instruction when He said, “Behold, you are made whole; sin no more, lest a worse thing come unto you.” Conceive of Him as saying this also to His confessor, “Lo you are made a confessor; sin no more, lest a worse thing come unto you.” Solomon also, and Saul, and many others, so long as they walked in the Lord's ways, were able to keep the grace given to them. When the discipline of the Lord was forsaken by them, grace also forsook them.
Source: The Epistles of Cyprian (New Advent)