St. Justin Martyr: On free will and responsibility

 But lest some suppose, from what has been said by us, that we say that whatever happens, happens by a fatal necessity, because it is foretold as known beforehand, this too we explain. We have learned from the prophets, and we hold it to be true, that punishments, and chastisements, and good rewards, are rendered according to the merit of each man's actions. Since if it be not so, but all things happen by fate, neither is anything at all in our own power. For if it be fated that this man, e.g., be good, and this other evil, neither is the former meritorious not the latter to be blamed. And again, unless the human race have the power of avoiding evil and choosing good by free choice, the are not accountable for their actions, of whatever kind they be. But that it is by free choice they both walk uprightly and stumble, we thus demonstrate. We see the same man making a transition to opposite things. Now, if it had been fated that he were to be either good or bad, he could never have been capable of both the opposites, nor of so many transitions. But not even would some be good and others bad, since we thus make fate the cause of evil, and exhibit her as acting in opposition to herself; or that which has been already stated would seem to be true, that neither virtue nor vice is anything, but that things are only reckoned good or evil by opinion; which, as the true word shows, is the greatest impiety and wickedness. But this we assert is inevitable fate, that they who choose the good have worthy rewards, and they who choose the opposite have their merited awards. For not like other things, as trees and quadrupeds, which cannot act by choice, did God make man: for neither would he be worthy of reward or praise did he not of himself choose the good, but were created for this ends; nor if he were evil would he be worthy of punishment, not being evil of himself, but being able to be nothing else than what he was made.

+St. Justin Martyr, from The First Apology, chapter 43.

St. Nicodemus the Hagiorite: Resistance to Sin

If you feel sometimes such a strong upsurging of sin that resistance to it will seem impossible and the very zeal to oppose it will appear exhausted, take care, brother, not to give up the struggle, but rouse yourself and stand firm. It is a subterfuge of the enemy, who, with the thought that resistance is hopeless, strives to undermine your firm stand and by making you lay down all your arms to force you to surrender to him. Make your mind see this subterfuge of the enemy more clearly and do not give ground. For so long as your will does not incline toward this passionate urge you are still among the victors, the fighters and slayers of the enemy, even if your sympathy is already ranged on the side of the passion. Nothing and nobody can force your will or steal victory from your hands an overthrow you against your will, no matter how obdurate and bitter the war waged in you by the enemies of your salvation. God endowed our free will with such power, that even if all a man's faculties, the whole world and all the demons rose up in arms against him and attacked him, they could not compel it. It is always left free to desire what they offer or demand, if it so wishes, or not to desire it, if it does not wish. On the other hand, for this very reason his will bears the responsibility for everything and is subject to judgment. Remember this well: no matter how weak and exhausted you may feel, you cannot find excuses for inclining toward a passionate suggestion. Your conscience will tell you the same. So the stronger the attacks the stronger the resistance you must prepare, and never abandon this resolve, repeating on all such occasions the word of command of one of our war leaders: "Watch ye, stand fast..., quit you like men, be strong" (1 Cor 16:13).
+St. Nicodemus the Hagiorite, from Unseen Warfare, chapter 14.

Note: Unseen Warfare was taken from a work originally entitled The Spiritual Combat, by the Roman Catholic scholar Lorenzo Scupoli. It was translated into Greek and edited by St. Nicodemus to conform to Orthodox teaching, and later revised by St. Theophan the Recluse who also renamed it. Thus, to some extent, each of these could be credited with the quotation.