Answer:
Christianity started at Pentecost by the power of the Holy Spirit on the apostles and disciples of Jesus Christ after His death. They spread His gospel throughout the known world approximately 2000 years ago.
Christianity, as far as this world is concerned, was manifested at the time Christ died on the cross. It was prophesied in the book of Isaiah.
Isaiah 55:11 So shall my word be that goeth forth out of my mouth: it shall not return unto me void, but it shall accomplish that which I please, and it shall prosper in the thing whereto I sent it.
Jesus Christ is what Isaiah is prophesying.
John 1:In the beginning was the Word, and the Word was with God, and the Word was God.
John 1:2 The same was in the beginning with God.
John 1:3 All things were made by him; and without him was not any thing made that was made.
John 1:4 In him was life; and the life was the light of men.
John 1:5 And the light shineth in darkness; and the darkness comprehended it not.
The Word entity of the Holy Trinity was manifested to the world in the likeness of men.
Philippians 2:6 Who, being in the form of God, thought it not robbery to be equal with God:
Philippians 2:7 But made himself of no reputation, and took upon him the form of a servant, and was made in the likeness of men:
Philippians 2:8 And being found in fashion as a man, he humbled himself, and became obedient unto death, even the death of the cross.
Philippians 2:9 Wherefore God also hath highly exalted him, and given him a name which is above every name:
Philippians 2:10 That at the name of Jesus every knee should bow, of things in heaven, and things in earth, and things under the earth;
Philippians 2:11 And that every tongue should confess that Jesus Christ is Lord, to the glory of God the Father.
"made in the likeness of men" is referring to his earthly existence. Man consist of three separate entities: soul, flesh, and spirit. Christ, being found in fashion of man also had three entities: soul, flesh, and spirit. But the difference is, His soul was not the same as ours. His soul was the "Word" entity of the Holy Trinity. So we find that Christ was truly a man; but not merely a man. Though He appeared as a man, His being was still Divine.
So, as Isaiah prophesied, the Word was sent forth. At the time Christ died on the cross His soul, flesh, and spirit separated. The terms relating to death in the Greek have a prefix "apo"; meaning to "separate". When this happened He cried out:
Matthew 27:46 And about the ninth hour Jesus cried with a loud voice, saying, Eli, Eli, lama sabachthani? that is to say, My God, my God, why hast thou forsaken me?
As his soul "The Word" was separating from His flesh and spirit, His human nature was crying out to God. At this point, the "Word" was coming into the world while His human flesh died and His human spirit returned to God. (see Ecclesiastes)
Now that the "Word" is in the world today, it is accomplishing what it was sent to do. The purpose is for the "Word" to take up residency in the souls of believers today. This is what being "born again" is all about. When the "Word" takes up residency in a human soul a transformation takes place. This is not just mere Christian rhetoric. This is a REALITY. The person becomes a new creation. As his soul becomes one with the Truth of the Word, old things pass away and all things become new.
To fulfill Isaiah's prophecy, the Word will prosper by entering more and more souls every day. It will not return void. It will return to God with the souls it resides in.
Being a Christian is much more than reciting the sinner's prayer.
Matthew 7:21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.
Matthew 7:22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
Matthew 7:23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
Not everyone waving a Christian flag is a Christian. But to those that truly believe from their innermost being, will see eternity in heaven.
----------------
Another Answer:
According to Matthew 16:18, Jesus said to Peter, "You, Peter, are the rock upon which I will build my church." However, scholars widely question the authenticity of this alleged quotation; and many scholars (even some who don't question its authenticity) question the meaning of this statement, since the Greek term, "ekklesia," which was used there for "church," signified, in that time, any sort of an assembly, even a political one; and a Jewish assembly (or - as it was then called - "sunagoge") was also a type of "ekklesia." Did Jesus start a Jewish sect? Christianity isn't that. So, this statement, even if it was authentic, doesn't answer the question: Who started Christianity - and when, and where, and why?
The only other Scriptural candidate for Jesus's having authorized Christianity is Matthew 28:18-20, in which the resurrected Jesus is quoted as ordering his followers, "Go throughout the world to make all peoples my disciples by baptizing them in the name of [the Trinity] the Father, the Son, and the Holy Ghost." However, this statement contradicts Matthew5:17-20, which quotes Jesus as saying "Do not think I have come to do away with the Law of Moses, ... for it will be eternally binding," and the first three of the Ten Commandments permanently ban any such thing as the Trinity, and clearly demand worshipping only the Father, never to include any second object. Even more emphatically, the Third of the Ten Commandments says "Do not take the name of God in vain," and so this alleged baptismal order was clearly in violation. Furthermore, the early Christian church didn't consider this alleged statement from Christ to be binding, and as late as the 16th Century this order was widely understood as having been directed only at Jesus's disciples in his own time, not at future generations, and the obligation was thought to have been fulfilled by them. In any case, the statement doesn't assert that a person who fails to comply with it will be viewed less favorably by God, or denied salvation. Moreover, only relatively recently did the statement come to be called "The Great Commission," and considered as the start of Christianity. This change of belief occurred at the time critical scholarship on The Bible first emerged, The Enlightenment. It's not how Christianity had seen itself during the religion's first 1,600 years.
And if Jesus didn't create Christianity, if a different person created it, then would Jesus have approved of what that individual was doing? Might Christianity even have been created by an enemy of Jesus? Not only might this have happened; it did happen.
The following press release summarizes the event that started Christianity: who did it, and when, and where, and why, and how: Did St. Paul Confess to Starting Christianity violating Jesus's Intent? New Book Says He Did.
How did Christianity start by worshiping a Jew while it negated and claimed to replace Judaism? Paul said in Galatians 2:16 "God approves only people who possess Christ-faith, never people who obey God's commandments." That doctrine is Christianity (salvation via Christ-faith) replacing Judaism (salvation via obeying God's laws). And yet Matthew 5:17-18 quotes Jesus himself as having said, "Do not think that I have come to do away with the Law of Moses. ... As long as heaven and earth shall last, not the least point nor the smallest detail of the Law will be done away with." Jesus was teaching Judaism, but Paul - who never even met Jesus - said in Galatians 2:16-21 that the death and resurrection of Jesus meant that obeying God's commandments was no longer the way to please God.
The subsequent conversation is being move to the discussion page.
Sum of Christianity was created in 1536.
Introduction to Christianity was created in 1968.
Mere Christianity was created in 1952.
The Essence of Christianity was created in 1841.
Practice in Christianity was created in 1850.
Foundations of Christianity was created in 1908.
Atheism in Christianity was created in 1972.
Center for Progressive Christianity was created in 1996.
Studies in World Christianity was created in 1995.
A New Christianity for a New World was created in 2002.
An Argument Against Abolishing Christianity was created in 1708.
Society for the Study of Early Christianity was created in 1987.