There were no Christians on earth until the Day of Pentecost when the Holy Spirit entered the disciples on that day. His entering was permanent at that time. The church and Christianity started at that time. This happened in Jerusalem 50 days after Jesus was crucified at the time of Passover.
The word "Christianity" isn't in the King James Version of the Bible. The term "Christians" was first used by people in the city of Antioch in reference to the disciples of Christ, and as a result of the work Paul did there [Acts 11:26].
"Christianity" is a general term coined by men, later, to label another of one of the world's many religions; in this case, one that touts the name of Christ. It's a "general term," in that there are hundreds of denominations and beliefs regarding Christ under this label, as the varied beliefs and sects of any other religion in the world.
What has come to be known in the world as "Christianity" is perhaps the very first thing about which Jesus warned His disciples:
"...the disciples came unto Him privately, saying, Tell us... what shall be the sign of thy coming, and of the end of the world [age]? And Jesus answered and said unto them, Take heed that no man deceive you. For many shall come in My name, saying, I am Christ; and shall deceive many." (Matt.24:4-5 KJV)
Paul fought off many attempts of Satan to deceive and pervert the churches he was planting among the Gentiles... and he struggled with it until he died, and was "taken out of the way." Until Paul was taken out of the picture... there was no multi-denominational religion known as "Christianity"... but the struggling Gentile churches that kept Paul busy... and that inspired his letters of instruction and teaching, that would one day become the meat and potatoes of the "New Testament."
"Don't be fooled by what they say. For that Day will not come until there is a great rebellion against God [within His church] and the man of lawlessness is revealed - the one who brings destruction... And you know what is holding him back, for he can be revealed only when his time comes. For this lawlessness is ALREADY AT WORK SECRETLY, and it will remain secret until the one who is holding it back [Paul's monumental efforts in and among God's church] steps out of the way." (II Thess.2:3-7 NLT New Living Translation)
There is no term of "Christianity" within the pages of the Bible. There is only Jesus', Paul's, and the other disciples' warnings to beware of people claiming to represent Christ... but who fail to obey and do the works of Christ:
"Pray, too, that we will be saved from wicked and evil people, for not everyone believes in the LORD. But the LORD is Faithful; He will make you strong and guard you from the evil one. And we are confident in the LORD that you are practicing the things we commanded you, and that you always will. May the LORD bring you into an ever deeper Understanding of the Love of God and the endurance that comes from Christ." (II Thess.2:4-5 NLT)