My apologies if this is an inappropriate/broad question, or if it has been asked before, but I stumbled upon this article, that reads,
The cross is a pagan symbol that was adored in Egypt thousands of years before Jesus was born. The Roman Catholic Church adopted the cross symbol at least 600 years after Jesus was supposedly crucified.
This reminded me of some other things I've read;
Many traditional elements of Christmas pre-date Christianity. In other words, Christmas was pagan before it was adopted (and renamed) by Christians.
...Christian saints and which Pagan Gods and Goddesses they are based on. [...] Often, information about Christianity is a major source of information about Pagan religions, and an understanding of the Pagan religions is quite revealing about the later monotheistic religions, too.
I have even seen this aspect of Christianization argued for Medieval literature;
Sometime in the eleventh century, two scribes preserved Beowulf in writing, creating the single manuscript of Beowulf we have today. The original poem and the poet were pagan, but the scribes were Christian and added Christian details to the poem: the poem now calls God the ultimate judge and ruler and even refers to events in the Old Testament.
These all allude to Christian saints, holidays, literature, and symbols all being of a Pagan origin. That being said,
I'm not sure what an answer to this would really look like, is "62% influenced by Paganism" the type of answer you are looking for?
I'm also not sure it's worthwhile to use a (very poorly designed) website designed to refute Christianity as a source on Christianity.
Christians were publicly using the cross as a symbol no later than 300 years after Jesus, probably privately using it prior to that. The historicity of "In hoc signo vinces" is not clear at all, but people were certainly talking about it less than 300 years after the crucifixion.
In terms of broad concepts, I would say in a large part it's true but irrelevant. Christianity didn't invent the concept of an afterlife, or the concept of a messiah, or the idea of death and rebirth. Christianity developed in the cultural context of Judaism and the Greco-Roman Middle East, so obviously it was influenced by that context. Rabbinical Judaism also developed from this cultural context. Christianity is extremely explicit about its Jewish influences, and many of the controversies within the early church both as detailed within the New Testament and as attested elsewhere are about balancing the Jewish heritage of Christianity and a desire to be accessible and open to the Greco-Roman population.
It's true that at least some and probably many stories of saints are influenced by pre-Christian stories and themes. Stories from the pre-modern era are often bent to fit familiar themes, this isn't at all unique to Christianity or stories of saints, people who recounted events at that time often felt like they should do so in a way that illustrates a lesson of some sort for example.
Paganism was always shunned by Christianity, despite having some overlapping concepts. A theme of the Roman persecution of Christians was trying to force Christians to participate in pagan ceremonies, which most would refuse. If became a significant controversy within Christianity whether or not to readmit Christians who had been forced to participate in pagan ceremonies. I'm not sure why it should be surprising that groups with overlapping concepts didn't like eachother., for example I'm sure you can imagine various groups who value the concept of "family" who all hate eachother.
During the post Roman period, the process of adopting concepts into Christianity was a lot different. It was usually a scenario where Christians had conquered a pagan area, or were trying to convert it, and came up with a teaching tool like "Oh, your story of your god of nature sounds a lot like the story of St. Bob, let me tell you about St. Bob". It wasn't as much wholesale adopting concepts but trying to make Christianity accessible to converts.
Many of the concepts of Christianity came directly from Judaism, rather than paganism, like for example the concept of a messiah.
Other concepts, like for example the concept of god or gods in general, or the theme of death and rebirth, existed in paganism, but they existed in so many distinct cultures that there is no original source. It's probably more of an anthropological question than a historical one why humans believe in god(s), or an afterlife, or things like that.