Lactantius lived c.250-c.325
Origen of Caesarea (c.185-c.253), in his work Contra Celsus (Celsus being possibly though not certainly an Epicurean (see highlight below)), touches on the 'able but not willing' formulation. Celsus' book does not survive except as quoted by Origen.
QuoteNow in our judgment God can do everything which it is possible for Him to do without ceasing to be God, and good, and wise. But Celsus asserts — not comprehending the meaning of the expression God can do all things — that He will not desire to do anything wicked, admitting that He has the power, but not the will, to commit evil. We, on the contrary, maintain that as that which by nature possesses the property of sweetening other things through its own inherent sweetness cannot produce bitterness contrary to its own peculiar nature, nor that whose nature it is to produce light through its being light can cause darkness; so neither is God able to commit wickedness, for the power of doing evil is contrary to His deity and its omnipotence. Whereas if any one among existing things is able to commit wickedness from being inclined to wickedness by nature, it does so from not having in its nature the ability not to do evil.
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The argument which Celsus employs against us and the Jews will be turned against himself thus: My good sir, does the God who is over all things know what takes place among men, or does He not know? Now if you admit the existence of a God and of providence, as your treatise indicates, He must of necessity know. And if He does know, why does He not make (men) better? Is it obligatory, then, on us to defend God's procedure in not making men better, although He knows their state, but not equally binding on you, who do not distinctly show by your treatise that you are an Epicurean, but pretend to recognise a providence, to explain why God, although knowing all that takes place among men, does not make them better, nor by divine power liberate all men from evil?
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But as he asserts that the Mosaic narrative most impiously represents God as in a state of weakness from the very commencement (of things), and as unable to gain over (to obedience) even one single man whom He Himself had formed, we say in answer that the objection is much the same as if one were to find fault with the existence of evil, which God has not been able to prevent even in the case of a single individual, so that one man might be found from the very beginning of things who was born into the world untainted by sin.