On the one hand; is pretty clear that mainstream, modern, materialism corrupts people - we can see this for ourselves, in our experience.
But on the other hand; Christianity does Not necessarily make someone a better person. Conversion may well make a person different, but whether he is better is not obvious. Indeed, Christianity is not terribly effective at preventing people from getting worse; at least, a lot of serious Christians seem to me to get worse in the same kind of ways that non-Christians are getting worse.
Quite a lot of Christians expect that Christianity will at least tend to make people better behaved, make them more likely to live by Christian morality. Maybe so. Sometimes it does; but often it does not - and perhaps especially not when institutional Christianity is (as now) declining, weak or itself corrupted and corrupting.
Why not? Well, in the first place, Christianity is primarily about immortal resurrected life after death; not about this life. So we shouldn't really be looking for its effects on here-and-now behaviour; except insofar as this is secondarily derived from a desire to repent and follow Jesus.
However, repentance is known only to God - or to put it another way, the evidence for repentance is known for sure only after someone has 'arrived in Heaven'.
Yet, clearly Christianity is about this mortal life as well - even if not in any kind of clear, one-to-one mapping of beliefs and behaviours. To discover how I think we need to return to the insight that mortal life was instituted by God, and entered voluntarily by each of us; in order that we experience and learn through living in this changing world; with the twin aims of:
1. Choosing to follow Jesus - by love, faith and trust - through death to life everlasting. And also:
2. To prepare for that Heavenly life - by 'theosis': that is, through becoming 'more divine', more 'Christ-like', or 'sanctified'.
In sum: what I think Christianity does do in mortal life is precisely to enable us to learn from our experiences.
We cannot learn from the experiences of this mortal life if we do not correctly understand the set-up of mortal life, what it is for, how it works. Materialism is false, therefore it prevents us learning.
Christianity - being true - is exactly the understanding we need in order to learn. Lack of Christianity (and especially its inverse, which is what Leftism is increasingly becoming) is to have a wrong understanding, and therefore to be unable to learn the true lessons of mortal living.
Learning must, in this, be distinguished from memory; we need to assume that learning is an objective process that affects us at a spiritual level and in a permanent way that cannot be effected by the changes and hazards intrinsic to mortal life.
If learning depended on memory, the benefits of a Christian life might be abolished by psychosis, dementia or a head injury.
So we need to posit that there is an indestructible kind of learning from life that is not bound to the body; and which is carried through the transformation that is resurrection. This was not a problem for our ancestors to understand, but we moderns often can't explain it - because we try to using the simplified models of science that exclude-by-definition just the kind of phenomena (such as soul or spirit) that does the job.
Anyway, assuming we can perhaps imagine that the experiences of life, including repentance of sins, can have a permanent effect; and be carried through the portals of death and the transformation of resurrection - then we can have a basic understanding of what difference Christianity makes to a person.
And we see that there is no symmetry between being, and not-being a Christian - each isn't a reflection, nor an opposite of the other...
Being a Christian does one kind of thing, and not being a Christian does another and different kind of thing.
Note added: It strikes me that this idea of Christianity enabling us to learn from the experiences of life is a plausible way of explaining some of those rare but remarkable behavioural transformations that do sometimes follow Christian conversion. For example when addicts, accountants or thugs suddenly repent and reform - and go on to lead a qualitatively better life. This may be understood in terms of acquiring a new ability to learn (by understanding the ultimate context of our specific behaviours). Without Christianity: (Proverbs 26:11) As a dog returneth to his vomit, so a fool returneth to his folly. But with Christian understanding, the individual may be able, for the first time, to learn from recurrent experiences of vomit-eating.