May people, including Christians, tend to assume that becoming a Christian makes you a better person. This is not true - or at least not necessarily true. The situation is more of a double-negative; that unless you become a Christian you cannot become a better person.
What Christian faith brings is not betterment but discernment. Christianity faith brings you a basis for discerning good from evil - which is necessary for you to learn from experience.
Or, to put matters more starkly; if you are not a Christian then (as Frank Berger has explained recently) you cannot learn from experience.
Knowing real-good from real-evil is necessary for real-learning, and without real learning you cannot get better.
Whether or not the Christian actually does get observably, objectively morally better in their behaviour; will depend on several factors - including their personality, upbringing and the society they inhabit.
But that is not the essence of Christianity. As often said: Christ came to save sinners - sinners are saved even if they do not become good. What is needed is 'repentance' - and repentance requires discernment: that is, knowing the difference between good and evil and identifying which is which. Then joining allegiance to good, and rejecting evil.
Modern Man, by contrast, often does not acknowledge evil - either rejecting the concept; regarding evil as relativistic, arbitrary, and socially-dictated; or equivalent to anything that causes suffering - especially of himself or people he likes; or else, in extreme and especially in the sexual realm, he inverts them - and regards his evil as what-used-to-be called good by Christians, and his new-'good' is whatever was disapproved or prohibited by Christians or traditional society.
In sum: Modern Man cannot, does-not learn, because he lacks discernment; and discernment can only come (for Modern Man) from truth, therefore from Christianity. And under modern conditions discernment must come from within each individual (obedience to institutional discernment will not suffice, and modern institutions are nearly-always corrupt and getting worse).
Discernment does not necessarily come from Christianity; but unless it comes from Christianity, it will not be real, powerful, or motivating-enough to enable that discernment which saves.