In order to "believe in Christianity," one must know the definition of "Christian!" According to Scripture, "...the disciples were called Christians first in Antioch." (Acts 11:26)
Today, the words Christian and Christianity are used to justify one's Christ-likeness, but are they really doing what the Word of God says in order to be Christ-like? Acts 11:26 tells us that "...the disciples" were called Christians.." Did anyone notice what happened prior to being called Christians? If not, let me remind you. On the Day of Pentecost (when the FIRST CHURCH was established), the Holy Spirit came (Acts 2:2) as Jesus promised (John 14:26) under the New Covenant (Jeremiah 31:33, Hebrews 8:10; 10:16). Thereafter, (and under the New Covenant in which we NOW live - Grace & Truth), the apostles ONLY baptized in the Name of the Lord Jesus! Do "Christians" do that? Acts 2:42 said (because Jeremiah & Hebrews said He would "...put his laws into their mind, and write them in their hearts;..." via the Holy Spirit (John 14:26) "And they continued in the apostles' doctrine." [paraphrasing] (Which was not the apostles' but Jesus' doctrine because He said He would "put His laws into their minds and write them in their hearts.") Has anyone "continued in the apostles' doctrine" which qualifies one for "disciple?" No! Most say "I don't go by what the disciples or apostles said, but Jesus!" Do they not believe Jeremiah and Hebrews? Did Jesus not say in Acts 9:15 that Saul (Paul) "...would be a vessel.." to Him? Did Jesus not tell Paul (via Holy Spirit) what to write to us for our salvation? Therefore, it was JESUS talking through Paul via the Holy Spirit!
Based on these Scriptures, do you still feel you are a Christian? Are you a disciple that follows what Jesus told Paul to tell us "After the Cross" under the New Covenant? Why make a New Covenant if no one is willing to follow? In I Corinthians 11:1 Paul said [paraphrasing] "...follow me as I follow Christ..." Revelation 21:14 says "And the wall of the city had twelve foundations, and in them the names of the twelve apostles of the Lamb." Are we not to build on that foundation? Otherwise, are we "really" Christians?