Humility keeps us cognizant of where we have power, and where we don’t – and why it’s useful to align ourselves with God’s will, according to Christianity.
On the other hand, Christianity emphasizes that we have free will, and therefore ultimate responsibility over our own actions.
The key here is that we have ultimate responsibility over our own actions. It also (correctly) notes that we have little ability to control anything beyond that. Instead, by choosing to do so, Christians can align themselves with God, who can then pull off amazing (‘miraculous‘) things beyond the power of any given human (or so the idea goes).
So there is an interesting balance, between us not being in control (God is) and us having ultimate control over our own actions. ‘Ultimate’ means that there is, in a moment, an ability to be self-conscious and control our action. However, in many cases we simply decide to do something (eat food that isn’t good for us long-term) because we decide to prioritize the moment, for whatever reason. What’s important to note here is that we can also begin to work toward changing the circumstances, such that some context doesn’t occur as much, or at all, say.
The goal in Christianity is to align ourselves with God’s will, but Christianity recognizes that free will isn’t sufficient. As noted, often we will prioritize short-term over long-term. This is where the development of will-power comes into play. There are various Christian practices aimed at increasing one’s will-power, and these are often called ‘ascetic‘ practices. The word ‘ascetic’ originally was applied to bodily training (for example, going to the gym). In the Christian tradition, it has come to be applied to spiritual training, and in particular to spiritual training aimed at increasing will-power.
Why is will-power important? You can think of the equation as free will x will-power = practical freedom. So, although we have ultimate freedom, often our will-power is so weak that we constantly make short-term decisions. It’s not that we don’t have the ability to decide, but that we haven’t trained our ‘spirit’ to make decisions that tend to benefit us long-term instead of short-term.
The first step in claiming the implicit power of free will is to simply be aware that it exists. Once one remembers one can choose, one can begin to put into place the actions which lead to things like increased will-power (through various spiritual practices, such as certain Christian ascetic practices).
(In this sense, the awareness of free will is the basis of all virtues, which are more or less habits of the mind, because free will ultimately is what allows us to make the decisions which might lead to the creation of those habits of the mind.)