This is a complex point that Kant makes in the Religion within the bounds of reason alone text, but the answer in that text is that for Kant once we have made a choice for evil, we can become incapable of later choosing for the good. People who have only read the Groundwork of the Metaphysics of Morals might be shocked by this as it seems to go against the idea at work there.
In part this is Kant's attempt to deal with disposition and habit. Habit is a topic he also worked on in the Metaphysics of Morals (not the Groundwork).
Looking just at the Groundwork, Kant seems to believe we are radically free as in we are rational and then able to act freely on our rationality. To do so is to make the maxim of our action correspond to the categorical imperative in its universality, treatment of humanity, and community of rational ends (inter alia). On this picture, we are not radically good or radically evil as nothing outside the will can affect the will. (This is a very Cartesian picture of the moral self -- and one that some sympathetic Kant interpreters (fan-fiction writers?) reject as being in the Groundwork).
Here, a major distinction is the third antinomy between the determination of the physical (phenomenal) world and the need for freedom and spontaneity for morality to possible and as a feature of our experience of ourselves.
Critique of Practical Reason presents a similar picture but a different argument for how we are capable of morally free choices. (The argument in the Groundwork is section III, a part where Kant tries to prove you can get moral freedom from the nature of freedom and rationality directly).
The rub is that it seems kind of clear that the choice to be moral is not equally accessible to everyone. Kant is well aware of this in his lectures on ethics (well before and continuing through to the Groundwork), his lectures and writing on Anthropology, his Metaphysics of Morals, and his Religion.
The Religion text tries to explain (a) what makes us do wrong, (b) how we can biased for or away being able to make moral choices, and (c) the role of God in the Kingdom of Ends (hint: God doesn't not provide moral laws to us except as a representative for the weak of moral reasoning). The first two questions are primarily dealt with in the beginning of the text.
The basic answer is that as animals we have three basic predispositions: (a) animality, (b) personality, and (c) humanity. The meanings of the last two terms is not as they might seem. Personality means that we are social. Humanity here is a synonym for rationality. As such, the predisposition to humanity is a predisposition to moral goodness.
Working against this, however, is that we have a Hang (propensity) to evil. The problem is that once we've given into this propensity, we become evil and the choice for good becomes more difficult, because our reasoning is no longer on a blank slate. Here, Kant distinguishes Wille and Willkur with the Wille remaining fundamentally free and able to make any choice (as on the Cartesian picture) but the Willkur being affected by the choices we have made and limiting the maxims presented to the self.
Kant claims that all of us have made the choice for radical evil. He uses in part in an empirical argument and in part points out the degree to which our predispositions can be manipulated to evil. (To get you more of the details I need to be in my office with my copy of Religion or open up some old papers).
This picture probably seems unconvincing to many readers today. To the astute reader, this picture should sound familiar. Basically, Kant has transposed the Lutheran account of moral responsibility and original sin (in this case original to all in that each sins) into his moral system.
Now, we can finally answer your question: for Kant, without God, people who are depraved cannot make the change for the better because the restriction of the will is not something they can overcome (since it affects their wills).
