Brand, Shiny, New Me or Gentile Me?

Ephesian's 4:17-24

Paul describes the moral condition of the gentiles as being hardened hearts that have given themselves over to sensuality so as to indulge in every kind of impurity, and then he reminds the church to put off there old selves, described above, and put on their new selves in Christ. Their new selves is a change in attitude to become like Christ in righteousness and holiness.

In vs. 20 I am confused as to how Paul arrives at this point. I don't understand what he means when after he describes the gentiles he says that they "did not come to know Christ that way." How would they have. Is this rhetorical? Meant to illustrate the deep divide between their old selves and new selves? Or is he saying that the following of our desires is not the way to know God, but rather being taught the truth is the way to the "new self". I am honestly looking for your thoughts and ideas to put this passage together.
I think that this battling hardening hearts, and struggling to put on a new attitude of the heart is one of our central lifelong challenges. I don't see how it ends or how there is one all-time cure that allows to never have to struggle with a hardening heart.

I can use the hardening heart, and the previously mentioned marks of maturity again to measure myself and my life by. This could create a set of questions that I regularly ask myself, in prayer, and use as a barometer of where I am at. Unfortunately they are not just yes or no questions and will require some work.