What does godliness look like? In describing godliness, we would probably come up with some sort of list, outlining characteristics and behaviours of a godly. It isn't difficult to cull several examples of this in Scripture, such as the the great commandments, loving God and loving others or the fruit of the Spirit: love, joy, peace, longsuffering, gentleness, goodness, faith, meekness, and temperance.
We could articulate all these factoids as a bullet point list of descriptive characteristics, and this would give us a rather comprehensive picture of the ideal behaviour of a Christian. Then this could serve as an apt description of Christ-likeness and could be represented in this way:
Godliness -----> -----> Descriptive List
The trajectory shown above treats the list as the product of who a person actually is. In other words, if a person is Christ-like, they will (increasingly / more or less) resemble the list. This would serve to assert that what you are determines how you act, which would be treating the list as descriptive, but there is a second trajectory that reverses this direction and suggests that what you do is actually godliness.
Godliness <----- <----- Prescriptive List
This makes the list prescriptive, suggesting that godliness happens when a person gets better at fulfilling this list. Articulated in this way, it might appear to be starkly wrong, but what happens if we look at how this practically plays out in our discipleship by considering the first discipleship relationship we have, our kids?
Imagine a parent who observes a pair of fighting siblings. How would we recommend that they approach the situation? One option is to take them to a list item and show them Jesus didn't do that, with appropriate admonitions that they should stop fighting, forgive, and live with patience, because that is what Jesus would do.
Two sets of parents each desire that their children lead godly lives, so they set out to raise them according to Biblical principles. One chooses a strict path and governs television, books, friends, and other activities to ensure that their children are well disciplined to measure up to their understanding godliness. The second parents does not want to be legalistic, like the first, so they don't have such a strict list. They are content if their children don't do drugs, get into immorality, or avoid other illegal activities and let the rest take care of itself.
In both cases, the parent's approach subtly teaches children that Christ-likeness is attainable by outward actions (some more/some less). Both are actually legalists (one just has a longer list of rules than the other), and neither deals with the heart issues like selfishness and pride that drive their sinful actions. Both approaches fall short in helping them toward hating their sin and loving what is right so that right actions result (the essence of repentance).
The bottom line is that God's rules don't change you, which means man's rules certainly can't change you. Instead, Biblical change is a heart operation of the Holy Spirit, showing the glory of God in the person of Christ by the revelation of His Word. This produces a Christ-like life that looks just like the godliness described in Scripture. But if in practice or in principle you reverse this trajectory of godliness, you have inadvertently adopted some form of legalism.