To exhibit the nature of defection or falling away, on the part of those who conduct themselves carelessly, it will not appear out of place to employ a similitude by way of illustration. Suppose, then, the case of one who had become gradually acquainted with the art or science, say of geometry or medicine, until he had reached perfection, having trained himself for a lengthened time in its principles and practice, so as to attain a complete mastery over the art: to such an one it could never happen, that, when he lay down to sleep in the possession of his skill, he should awake in a state of ignorance. (Book I, Chapter 4, Part 1)

I expect the analogy will be given a spiritual application.

One might awaken ill, still terribly tired, or deeply depressed. In any of these conditions the skill might persist but be applied haphazardly.

No matter how great our mastery, if we have a stroke we may awaken stripped of our prior skills.

But besides all this, I am uncomfortable with the implication that the spiritual person "trained himself."

There is a place for self-discipline. But much more fruitful is opening to God's love, surrendering to God's purpose, and adapting to God's unfolding reality.