But, on the other hand, those who in no wise complete the good things they have begun are to be admonished to consider with cautious circumspection how that, when they accomplish not their purposes, they tear up with them even the things that had been begun. For, if that which is seen to be a thing to be done advances not through assiduous application, even that which had been well done falls back. For the human soul in this world is, as it were, in the condition of a ship ascending against the stream of a river: it is never suffered to stay in one place, since it will float back to the nethermost parts unless it strive for the uppermost.
If then the strong hand of the worker carry not on to perfection the good things begun, the very slackness in working fights against what has been wrought. For hence it is that it is said through Solomon, He that is feeble and slack in work is brother to him that wasts his works. For in truth he who does not strenuously execute the good things he has begun imitates in the slackness of his negligence the hand of the destroyer. Hence it is said by the Angel to the Church of Sardis, Be watchful, and strengthen the things which remain, that are ready to die; for I find not your works complete before my God .
Thus, because the works had not been found complete before his God, he foretold that those which remained, even such as had been done, were about to die. For, if that which is dead in us be not kindled into life, that which is retained as though still alive is extinguished too. They are to be admonished that it might have been more tolerable for them not to have laid hold of the right way than, having laid hold of it, to turn their backs upon it. For unless they looked back, they would not grow weak with any torpor with regard to their undertaken purpose.
Let them hear, then, what is written, It had been better for them not to have known the way of righteousness than, after they have known it, to be turned backward. Let them hear what is written; I would that you were cold or hot: but, because you are lukewarm, and neither cold nor hot, I will begin to spew you out of my mouth. For he is hot who both takes up and completes good purposes; but he is cold who does not even begin any to be completed. And as transition is made through lukewarmness from cold to heat, so through lukewarmness there is a return from heat to cold.
Whosoever, then, has lost the cold of unbelief so as to live, but in no wise passes beyond lukewarmness so as to go on to burn, he doubtless, despairing of heat, while he lingers in pernicious lukewarmness, is in the way to become cold. But, as before lukewarmness there is hope in cold, so after cold there is despair in lukewarmness. For he who is yet in his sins loses not his trust in conversion: but he who after conversion has become lukewarm has withdrawn the hope that there might have been of the sinner.
It is required, then, that every one be either hot or cold, lest, being lukewarm, he be spewed out: that is, that either, being not yet converted, he still afford hope of his conversion, or, being already converted, he be fervent in virtues; lest he be spewed out as lukewarm, in that he goes back in torpor from purposed heat to pernicious cold.
Source: Pastoral Rule (New Advent)