In MSLN, hardening is the chief danger. One meaning of "calculus" is "hardened deposits" -- a spiritual danger. The habits and loves we have for idols of whatever sort (or pick another analogous word if you think "love" is too sacred for what we feel for idols) are attachments that can compete, perhaps fatally, with our love for God. They accumulate, and may have to be forcibly removed, by us or by God, if they threaten our salvation. Perhaps they can be prevented with some kind of hygiene (like with calculus on teeth).
If you have something in your life in addition to God, it might not be bad for you. There are many things which are good other than God (in some sense of the word "good"). If they don't fatally compete with love of God, they are basically safe. But if those attachments are hardened (fixing to be irrevocably necessary to you), then they must someday be unhardened or removed.
But another meaning of "calculus" has to do with measuring rates of change and is considered below.
The following paragraph contains a brief explanation of calculus, using position, velocity, and acceleration as examples. There are better/fuller explanations elsewhere if this isn't good enough.
In physics, we can measure a particle's position. Then, if the particle's position is changing, it has a velocity. The velocity is the rate of change of the position. If the velocity is changing, it is accelerating or decelerating (has a positive or negative, nonzero acceleration). The acceleration is the rate of change of velocity. We use the mathematical tools of calculus to calculate the derivative of position, and this gives us the velocity. This is the first-order derivative of position. Then we can take the derivative of velocity, the derivative of the derivative of position. This is the second-order derivative of position, acceleration. And we can go on to higher-order derivatives of position, like the rate of change of acceleration (third-order), or the rate of change of the rate of change of acceleration (fourth-order), and so on.
(Thinking spiritually:) You can think of yourself as traveling on a path toward full kinship with God, holiness, being in tune with him. You move closer and sometimes further from him. If your position ever reaches your destination, then you are safe. But until then, if your velocity reaches 0 (you stop moving toward God), and never increases (it is never saved by acceleration), then you are lost.
The world judges people based on position. And it's right to do so. Position enables you to be trustworthy, and the world does not want betrayal. Untrustworthiness leads to unnecessary harm. But this can be deceptive. We forget that it's velocity that's really critical, and a further variable that may or may not be well captured by mathematics, one of "that-which-brings-things-to-completion" ("endurance"? "ultimation"?)
In your spiritual life, you will have position yelling at you to pay attention to it, but velocity may be quieter. Acceleration is even quieter, and the higher-order derivatives of acceleration quieter still. There are an infinite number of orders of derivatives of position possible to compute, and they get harder and harder to sense as you get away from position. But it's certainly useful to think about velocity and acceleration, and perhaps develop a sense for the derivative of acceleration.
If possible, try to remember that other people aren't just position, but are also velocity, and acceleration.
The previous metaphor may seem to suppose that progress is linear. But that may not be realistic. Another metaphor: You may be moving toward or away from God in some kind of 3D space. Up until recently, you may have been traveling basically straight toward him (straight-ish but with some error, straight as far as you can see from a distance), but because you were off by some angle, you need to make a course correction as you get closer. Maybe you have to divert around an obstacle that blocks your path, and actually move away from God for a time in order to proceed down your path toward him. (It's dangerous to desire or be comfortable with any movement away from God, but that movement may be what your life ends up bringing you.) In this picture, progress can be a spiral or some wandering path. But that "endurance" or "ultimation" variable still matters. You could think of it like, its health and strength, and the direction its health and strength are headed, are like velocity, and acceleration (respectively) in the other example.