15 That which is crooked cannot be made straight: and that which is wanting cannot be numbered.
[AD 379] Basil of Caesarea on Ecclesiastes 1:15
He is upright in heart who does not have his mind inclined to excess or to deficiency but directs his endeavors toward the mean of virtue. He who has turned aside from valor to something less becomes crooked through cowardice, but he who has strained on to greater things inclines toward temerity. Therefore the Scripture calls those “crooked” who go astray from the middle way by excess or by deficiency. For, as a line becomes crooked when its straightforward direction is deflected, now convexly, now concavely, so also a heart becomes crooked when it is at one time exalted through boastfulness, at another dejected through afflictions and humiliations. Wherefore Ecclesiastes says, “The crooked will not be kept straight.”

[AD 399] Evagrius Ponticus on Ecclesiastes 1:15
The number by which God numbers the saints displays a certain and determined spiritual order, as it is said, “He numbers the multitudes of stars; and calls them all by name.” … Now, if David says, the understanding of God is without number, it is not as though it were unworthy of God’s essence that it cannot be numbered, or because the nature of such a number cannot be comprehended. For, just as the word invisible has two meanings: first, when applied to something that by its very nature is invisible (e.g., God), and second, when applied to something that may be visible yet is not normally seen, like the ocean floor because it is hidden under waters; similarly, “innumerable” has two meanings: what cannot be numbered by nature and what cannot be numbered for some other reason.

[AD 420] Jerome on Ecclesiastes 1:15
"A
twisted thing cannot be made straight, and what is not there cannot be
numbered. "Whoever is wicked cannot be corrected,
unless he was corrected beforehand.
Anything that is already correct will receive embellishment; and that
which is deviated will receive correction.
A man is not called wrong unless he has been diverted from the correct
path. This is contrary to the heretics,
who entertained certain characteristics, which do not seem to be sane. And since what is missing is lacking, it
cannot be numbered. Besides, only the
firstborn of Israel were counted. The
women, slaves, children and the people from Egypt, although of a great number,
were largely overlooked, being referred to as a reduction from the army,
without a number. The meaning of this
can also be: such wickedness is done in the sphere of the world that the world
is scarcely able to return to its completely good condition; nor is it able to
regain easily its order and complete state, in which it was first created. Another meaning of this is: when all men have
been restored to goodness through repentance, only the devil will remain in his
wickedness. For all things which are
done under the sun are done by his will and in the spirit of malevolence, while
sins are piled on sins at his instigation.
Then it can also mean: so great is the number of deviants and of those
who have been taken away from God's flock by the devil that it is impossible to
count them.