Paul made it easy for the pagans by throwing away Jewish practices that Jesus, as a good Jew, followed, including circumcision, kosher dietary laws, and Sabbath observance. Paul also made G-d into a human being (Jesus), again to make things easy for the pagan idol worshipers to understand.
Jesus never viewed the Old Covenant as abrogated or obsolete and followed it rigorously as an observant Jew. Jesus did not want to die, asking G-d why He forsook him. On the crucifix, Jesus referred to G-d as "my Father", which is one of about ten ways that Jews routinely refer to G-d, not indicating any actual parental relationship.
The Romans killed Jesus because he wanted to be Messiah, which meant becoming the next king of the Jews and kicking out the Roman's appointed puppet king for the Jews, so the Messiah thing was treason against Rome and punishable by death.
Many Jews wanted to be Messiah and all of them failed. The Romans crucified many thousands of Jews according to historians of that time, primarily because the Romans feared that the Jews would rebel, which they did twice, resulting in millions of Jewish deaths, the eviction of the Jews from Judaea (Israel) and the erection of the Arch of Titus in Rome directly in front of the Roman Senate to celebrate the Roman victory over the Jews.
Later on, Jews changed the idea of Messiah to apply to whomever would bring world peace and Paul changed the definition to be whomever gets you a free ticket to heaven.
Because there is not and has never been world peace, Jews do not believe the Messiah has yet arrived. Because Christian's accept Paul's definition of Messiah and because Christians believe that they have a free ticket to heaven, Christians do believe that the Messiah has arrived.
The key problems with Christianity include:
1) pathological, genocidal anti-Semitism (they stole the Jews' religion, or tried to, and so wish to bury the evidence...literally)
2) non-obligatory nature of keeping the Ten Commandments (since you can always accept Jesus on your deathbed and go to heaven)
3) perverting the idea of Original Sin to be that which guarantees anyone who doesn't figure out some trick to be saved will burn in Hell for eternity, rather than the metaphorical event that gave humankind Knowledge (and the moral responsibility that comes with it) that made our species different from the animals
4) making G-d fallible enough to create a moral law-based path to Heaven (Judaism) and replacing it later with a free ticket to Heaven (Christianity)
5) making G-d egoistic enough to sacrifice his son for Man's salvation, but only allowing those who believed that such an incident occurred to benefit from it
6) making the focus of life into the after-life, rather than this life, thereby devaluing G-d's gift of Creation
7) numerous genocides, inquisitions and persecutions throughout history and throughout the world
Christianity first became popular when the unpopular Nero fed them to lions in the Roman Coliseum, after offering them a choice of renouncing their faith; Romans hated Nero, after he allowed the city to burn while he fiddled (and due to his cruelty, insanity and sexual perversions), so they took an interest in this Christian demonstration of resistance to his scape-goating them for the fire.
Christianity's second major advance occurred with the adoption by Constantine of Christianity, following his wife, and his imposition of it as the official religion of the Roman Empire. Many argue this was to appeal to the powerful urban elite, many of whom had become Christian, some of whom had been converts to Judaism, but switched to the easier Christianity after the Jewish Revolts failed and Roman persecution of Jews became severe.
Christianity's final paganization occurred at the Council of Nicea, where pagan priests, fearful of losing their jobs after Constantine's conversion, became Christians but imposed their paganized views on the young faith, thereby elevating Jesus to the status of G-d and introducing Jesus-idols into churches. This culminated the process of de-Judaizing Christianity and resulted in the Iconoclastic controversy, where the pagans proved victorious.
Having succeeded in eliminating any part of a Jewish soul or moral compass from the faith, Christianity then began a 1,500-year murder spree, destroying indigenous cultures throughout Europe, the Americas and Africa, resulting in a Christian population today of close to two billion.
Today, in its heartland of Europe, Christianity is in decline. In England and France, more worshipers attend mosques than churches. In the Middle East, most Christians have been evicted or killed by the Muslims, except in Egypt, where Christians are merely persecuted. However, Christianity is succeeding in the Americas, Africa and Asia (the latter in which it fills a religious vacuum).
I, however, who worship Christianity, will enjoy an eternity of bliss in Heaven, perhaps playing harps, perhaps visiting dead friends and relatives whom I miss, and perhaps flying around with a new set of angel-wings."
Non-Christian: "Dude, you're one wacked-out freak. Get a life."
There are many different denominations of Christianity, and it is incredibly hard to estimate how many Christians there are in the United States because there is no common belief on what defines a Christian. The most liberal view of everyone who says they are a Christian being a Christian would place the figure around 75%, whereas the considerably more conservative belief that anyone outside your own denomination is not a Christian can place the figure as low as 0.1%.
Denominations sprout from the original idea of Christianity because some phrases from the bible contridict each other, and thus to harmonise this people say phrase A is literal whereas phrase B is symbolic. However, one group might say A is symbolic, but another might say B is symbolic. Thus, there are now hundreds of different denominations preaching slightly different versions of effectively the same source material.
Unfortunately, as Christianity is the 'big' religion of the Western World, it gets a lot of negative press. However, like all groups, Christianity has the best and worst among their number. The Christians who get the most press are the ones who give the stereotypical image of never doing anything wrong, and sitting on streets shout 'REPENT OR GO TO HELL', or something similarly pleasant.
However, many Christians - most, in fact - are decent people who just want the right to practice their own religion, like everyone else (or the right to NOT practice a religion, in the case of some).