This is a somewhat complicated question.  About the spread of Christianity, I assume you are primarily talking about the time of Western imperialism, when the Spanish, Dutch, French and English conquered massive parts of the world, bringing many peoples into political subjection, but also bringing one form of Christianity or another to huge parts of the world.  The legacy of this process is both very good and very bad.  You seem aware that many or most of the religions of these conquered areas were abhorent forms of worship, including human sacrifice and offerings to false gods.   From a religious perspective (assuming as I do that Christianity is the true and correct form of religion), the net result of colonization appears quite positive, at least in this sense that it eliminated abhorent false religions and brought knowledge of the true God to pagans!   But, in the process many beautiful cultures and languages were mercilessly destroyed, and tens of millions of native peoples lost their lives.   Religious change was overtaken by financial colonization and destruction, not only of ungodly, pagan religious practices, but also of much which was beautiful and praiseworthy.  As I have said, the legacy of the “Christianization” of the world by the western imperial powers is a very mixed one–combining the very good with the very bad.  As disciples of Jesus, we need to be honest about the horrors of imperialism, and humble about taking the “blame” for these evils as Christians, especially if we are “Western” Christians.  Any joy at the Christianization of the world ought to be tempered with humility and acknowledgement that evil was done in the name of good.

You say that these cultures were destroyed and replaced with Christianity.  This is a simplification.  Their cultures were largely destroyed in many cases, and completely destroyed in others, but they were not so much replaced by Christianity, as they were replaced by Western culture and Western, “modern” ideas about government, economics and so much more.  Christianity came along as a by-product of this cultural and economic transformation.  It is part of the thing which replaced these other cultures, but it is not the chief thing which the imperialists brought with them.  To the extent that they brought Christianity, to that extent, their effect was positive, in my view.

Did these other cultures and religions uphold good morality?  I would say that this, too, is a mixed bag.  All religions hold to at least some concept of morality, and many of them have a kind of morality with a fair amount of parallel to the Christian idea of moral truth.  This is not a complete shock, as God created all of us in his image, and he gave all of us a conscience.  So, there is some overlap in the morality of the cultures and religions which were replaced by Western culture and Christianity.  However, I believe that, in principle, the Christian morality which was spread by the Europeans was a better morality than the ones that were replaced.  I can give many examples, such as the importing from Europe and from Christianity the abolition of slavery, the idea of personal rights, of human freedom, and science.  All of these things were the direct result of Christian influence, and they have, in many cases brought an improved morality to the conquered states.  But… as I said, this is a mixed bag.

Atheists have tried, but largely failed, to prove that morality is the result of evolution.  Some aspects of unselfish behavior might benefit the overall population among naturally social species, but it is a HUGE stretch, not supported by data or by common sense, to claim that all the Christian ideas about morality were produced by natural selection.  Many aspects of moral truth in Christianity certainly would not have evolved by natural selection!  In fact, selfishness, and a willingness to take physical advantage over rival groups are strongly “positive” traits, as far as natural selection is concerned.  This claim is a VERY weak one, which is not supported, either by experiment or by common sense analysis of human behavior and of Christian morality.

Did these people not care?  I am sure that some did and some did not.  People are the same, more or less, everywhere.  Human-created religions are, at least in part, humans’ attempt to do what is right. But these societies and cultures, outside of Christian or Jewish influence, were always inferior in their morality to that of Christianity.

John Oakes