One may conjecture the following possibilities. The Apostle freed them from all the bonds of the elemental law, but with the condition that they live in the Spirit. It is possible that someone used this freedom like the others, but did not care for life in the Spirit. He regarded it as something abstract and not very necessary.
It is also possible that another, while recognizing the necessity of spiritual life for Christians, limited it to abstention from gross vices and the performance of some visible good deeds. He did not give strict attention to the purification of his heart.
In both cases moral laziness is assumed: unwillingness to compel oneself, to make an effort, to labor with mind, heart, and body. It rests on the false hope that what exists will pass as sufficient, since something does exist.
The Apostle says: no, do not deceive yourselves with such empty hopes. God cannot be deceived. He sees everything: your laziness and your unscrupulous attitude toward the work of your salvation. You will be condemned for the very fact that you do not do everything that depends on you. You dig in your heels and make excuses as soon as any sacrifice or effort is required, whether bodily or spiritual.
For this you will hear from the righteous Judge: evil servant! You will receive the sentence given to the slothful man who hid the talent. The Apostle draws this from the general law: what you sow, you will also reap.
Yet the thought is also visible here: what you sow, however you hide it and however you color it, will in due time come into the open. It will serve either for your justification or for your accusation. Only true fruitfulness will receive the bright reward.
Everything that seems good but is not truly so will not stand in its disguise before the light of God's all-penetrating judgment. What a man has sown in his nature, what he is according to the sum of his whole life and activity, such he will be recognized to be. According to this his true worth will be determined. Such a place he will receive, and the corresponding measure of either blessings or punishments. This is what he will reap, and what he will go on reaping for all eternity.
“The God of all watches over everything. Do not think that He does not know what is done. But know that the harvest will correspond to the sowing. What you sow, you will also reap” (Blessed Theodoret).
“God knows your hearts. Your powers and means are not unknown to Him. A plausible self-excuse may satisfy a man, but it cannot deceive God” (Blessed Theophylact).