c. 426–429 · John Cassian, Conferences (Conlationes) 9.34
For every one that asketh receiveth, and he that seeketh findeth, and to him that knocketh it shall be opened;” and again: “All things whatsoever ye shall ask in prayer believing ye shall receive, and nothing shall be impossible to you.” And therefore even if all the grounds for being heard which we have mentioned are altogether wanting, at any rate the earnestness of importunity may animate us, as this is placed in the power of any one who wills without the difficulties of any merits or labours. But let not any suppliant doubt that he certainly will not be heard, so long as he doubts whether he is heard. But that this also shall be sought from the Lord unweariedly, we are taught by the example of the blessed Daniel, as, though he was heard from the first day on which he began to pray, he only obtained the result of his petition after one and twenty days. Wherefore we also ought not to grow slack in the earnestness of the prayers we have begun, if we fancy that the answer comes but slowly, for fear lest perhaps the gift of the answer be in God’s providence delayed, or the angel, who was to bring the Divine blessing to us, may when he comes forth from the Presence of the Almighty be hindered by the resistance of the devil, as it is certain that he cannot transmit and bring to us the desired boon, if he finds that we slack off from the earnestness of the petition made.