Struck from within and from without, and no longer having support in the man, the passions soon yield. They begin to weaken and withdraw. The striking from within is more essential in the work of spiritual warfare. Yet it is more subtle, and easier for the attentive person. The striking from without, through doing good or performing deeds opposed to the passions, is harder. It requires the exertion of the powers of soul and body, and the overcoming of outward opposing circumstances.
Therefore, after speaking generally about sowing in the spirit, the holy Apostle dwells especially on that side of sowing which reaches the same end through doing good. He does this to inspire labor and to forestall self-indulgence and slackening. From every good deed a seed comes into the spirit, as from every piece of food a small atom enters the common mass of nourishing juice. The more good deeds there are, the more successful the sowing in the spirit. The sooner the good dispositions formed in the spirit mature, the more ready the field of the spirit is for harvest.
He speaks as though saying: As for doing good, see to it that you miss no occasion for it. When you do the good that lies before you, do it with zeal and joy. Do not be burdened, crushed, or murmuring. Nothing of what is done will perish. In its own time it will give its fruit, and you will certainly reap it.
Blessed Augustine sees this aim in the Apostle's words, namely encouragement toward the uninterrupted doing of good. "To sow what is good, that is, to do good, is easier than to remain in it unchangingly and constantly. Fruit that is visible and near at hand inspires labor. But our harvest is promised at the end. Therefore encouragement toward constancy and firmness is necessary. This is what the Apostle does when he says: While doing good, let us not grow weary. For only he who endures to the end will be saved (cf. Mt 10:22). The Prophet also teaches: Wait for the Lord; be of good courage, and let your heart be strengthened; and wait for the Lord (cf. Ps 26:14)."
The same thought is found in blessed Jerome. "Here the Apostle exhorts to constancy in zeal for doing good those who would still like to see the reward for good in this life. They do not know that, as in everyday life there is one time for sowing and another for harvest, so in the spiritual sense the whole present life is appointed for sowing good deeds. The future judgment will be the harvest. Then, according to the quantity and quality of the sowing, each one will have a fruitful harvest: some a hundredfold, some sixtyfold, and some thirtyfold."
All our interpreters refer the words "not growing weak" to the harvest. They understand them to mean that this harvest will no longer require labor and exhaustion, but will come of itself. Thus Saint Chrysostom says: "What he writes about this in other Epistles he says here also. One must not only give alms, for example, but give willingly and continually. He indicates this with the words, to sow, and, not to grow weary. But after demanding much from them, he places the reward at the doors for them. He points to a certain new and extraordinary harvest. In agricultural labors, not only the sower but also the reaper endures great weariness, struggling with heat, dust, and the great difficulty and burden of the work. But then, he says, there will be nothing of this kind. This is clear from what he said: For in due time we shall reap, not growing weak. By these words he persuades and draws them toward doing good."
Thus blessed Theodoret says: "Let nothing grievous interrupt zeal for the good. For we shall reap what has been sown without labor. The Apostle expressed this by saying: not growing weak. With sensible seeds, both sowing and harvest are joined with labor. Often the heat of harvest time weakens the reapers. But this harvest is not such. It is free from labor and sweat."
The same is said by Saint Damascene, and by Oecumenius with Theophylact. Only the opinion of blessed Photius, cited by Oecumenius, is this: "Or, not growing weak, μη εκλυομενοι, means not becoming lazy and not falling away from doing good." That is, we shall reap in due time if we do not weaken and become lazy in doing good. Jerome also understands it thus.
Neither interpretation contradicts the flow of thought. But the first is closer and corresponds more fully to the Apostle's aim, which is to inspire constancy in good. The grammatical structure of the speech also inclines one toward it.