I agree that Christianity is powerful, popular, and influential. If the question is whether or not the Christian Faith is relevant to politics or society or culture then I would agree. I would agree in the same right that the Bible is relevant to the study of Western literature just as the Communist Manifesto is to Russia. However, if the question is whether the religion is relevant to morality, theology, philosophy, and ethics than my response is a resounding no, or in this case I agree that it is irrelevant. Christendom should be left to the history books, not staples of modern morality. I have previously posited a theory that you cannot me a moral person and follow an Abrahamic religion (Islam, Judaism, Christianity, etc.) since they follow a God who wrote a book. To me, this god either completely wrote it or wrote none of it, since why allow your word to be officially tainted and impure? So a moderate or liberal or loose constructionist Christian will say that they believe Jesus rose after three days, that god created them in his image, and that if they are good anyone can go to heaven. But they disregard all the things about sexism, homophobia, war on nonbelievers, murdering children, and valuing rape as outdated and only relevant when it was written. If even one part of the bible is irrelevant or immoral than either god didn't write it or he doesn't deserve to be worshipped. If you are a literalist or fundamentalist then you believe everything in the bible is true and should be adhered to. In this case, the person is a true as can be Christian in regards to the Old and New Testament but they are not moral by their nature of supporting such atrocious deeds the Bible deems appropriate (genocide, child sacrifice, xenophobia, sexual repression, genital and bodily mutilation, etc.). Once more, understanding Christianity is integral to understanding western history and contemporary culture, the same for Islam and the Middle East and Northern Africa. However, in regards to morality, the Bible and Christianity support discrimination, favoritism, war, oppression, and discrediting the value of the world we live in.
Since Jesus of Nazareth was a real person, and since people from the beginning of year one people looked to Jesus as a person who preformed miracles, I'd say Christianity is as relevant today as it was when it first began. There is no other religion like Christianity. Jesus was and is someone that everyone can look up to as a role model.
About one in every three people on Earth are Christian, compared to the hundreds of different religions and belief systems which exist. The Christian Religion is still used as much as ever in many ceremonies such as Marriage, and Governmental ceremonies such as the coronation of a new monarch in England. In the US, try running for president on a platform that looks down upon and shuns Christianity. You can find a church with a congregation that meets weekly in nearly every city and town on the entire planet. Christianity is THE largest and most influential religion that man has ever known it is far from irrelevant.
Though I am not practitioner of such a belief system I do know that for many people a belief system is a way of solace and grounding to the world, as we all have our ultimate concerns. I agree that Christianity should not be making a large influence in our political or socioeconomic futures (at least not in the USA), but to call it irrelevant is naive at best. I truly believe that had Christianity not risen at the time they did, we would be discussing other religions or even the old Paganism as irrelevant.
Summary: Keep your religious systems in your own home, and conduct your time in politics and social justice as a human being bettering human beings rather than your religious title.