This is a great question. Some historical context is important. Martin Luther was a theologian and one of the people who started the Protestant Reformation in Europe. During this time, he had many controversies and one of them was with Erasmus. It concerned the topic of freewill.
Martin Luther's little book, Bondage of the Will, argues that due to sin, that is, the fall of humanity in Adam (Genesis 3) that the human will is now bound by its fallen nature. Therefore, the human will is not completely free. It cannot choose righteousness. To put it another way, we are freely able to choose whatever sin we want to commit. He writes:
"For if man has lost his freedom, and is forced to serve sin, and cannot will good, what conclusion can more justly be drawn concerning him, than that he sins and wills evil necessarily?"
He again says:
"All the passages in the Holy Scriptures that mention assistance are they that do away with "free-will", and these are countless...For grace is needed, and the help of grace is given, because "free-will" can do nothing."
As one can see, Luther's theology emerges from his interpretation of the New Testament.
“Man is like a horse. Does God leap into the saddle? The horse is obedient and accommodates itself to every movement of the rider and goes whither he wills it. Does God throw down the reins? Then Satan leaps upon the back of the animal, which bends, goes and submits to the spurs and caprices of its new rider… Therefore, necessity, not free will, is the controlling principle of our conduct. God is the author of what is evil as well as of what is good, and, as He bestows happiness on those who merit it not, so also does He damn others who deserve not their fate.” (‘De Servo Arbitrio’, 7, 113 seq., quoted by O’Hare, in ‘The Facts About Luther, TAN Books, 1987, pp. 266-267.) All these passages come from a tract Luther penned, titled, ‘De Servo Arbitrio ,’ or ‘Bondage of the Will,’ in which the great reformer works hard to present the case that free will does not exist. Scripture, of course, disagrees, in both word and spirit. In Sirach 15:11-20, we find: “Say not: ‘It was God’s doing that I fell away’: for what he hates he does not do. Say not: ‘It was he who set me astray’; For he has no need of wicked men… When God, in the beginning, created man, he made him subject to his own free choice. If you choose you can keep the commandments… There are set before you fire and water; to whichever you choose, stretch forth your hand.” So you see, the scripture is quite clear on the matter: “When God, in the beginning, created man, he made him subject to his own free choice.” But, you object, Sirach is ‘apocryphal’ — Luther discarded it, questioning its canonicity. And no wonder, we respond, considering how directly it confutes his teachings. But we can also point to Deut. 30:19-20, in which God tells us: “I have set before you life and death, the blessing and the curse. Choose life, then, that you and your descendants may live, by loving the Lord, your God, heeding his voice, and holding fast to him.” So we see that man is more than simply free to choose; he is obliged to choose. And earlier yet, in Gen. 4:7, God speaks to Cain: “Why are you so resentful and crestfallen? If you do well, you can hold up your head; but if not, sin is a demon lurking at the door: his urge is toward you, yet you can be his master.” And, finally, in John 15:15, our Lord pronounces his love for us, his followers: “I no longer call you slaves, because a slave does not know what his master is doing. I have called you friends…” Hardly sounds like the words of a rider to his horse. As often happens, Paul has the final word: “But if, in seeking to be justified in Christ, we ourselves are found to be sinners, is Christ then a minister of Sin? Of course not!” (Gal. 2:17.) A more direct contradiction of Luther’s pronouncement, “God is the author of what is evil as well as of what is good,” is difficult to imagine. Luther’s position includes no accountability. No responsibility. No sense of learning or of being perfected through the course of our lives. No dignity even. Just the bleakest, most oppressive coercion which robs human life of any meaning whatsoever. What you do in your life — even the love you evidence toward your neighbors — means nothing, according to Luther. Your struggles, your suffering, your perseverance — none of it amounts to anything. Your will is not even in your own hands.