What is Christianity? How could someone say that he/she is a Christian?
These questions were already answered to me by pastor when I was beginning my path towards God. But then just a while ago in school, in our World Religions subject, our teacher asked on what do we think is the religion with the largest number of adherents. Then a flow of answers came up - Roman Catholic, Islam, Iglesia ni Cristo, Jehovah's Witnesses, etc. Then our teacher stopped my classmates by saying that the Roman Catholic, Iglesia ni Cristo, Jehovah's Witnesses (and other more) are all same because they're all under Christianity.
John 1:12 was taught to me, explaining that those who believe, follow, have repented, and thus having a personal relationship with Christ are those that are called Christians. And that Christianity is not just "
the religion based on the person and teachings of Jesus of Nazareth, or its beliefs and practices".
I am not here to hate but to ask to all of you for your insights and that I may be enlightened. Thank you and God bless! 
There are a number of issues that are raised in your post that need clarification in order to sensibly answer your question.
Firstly the definition of what church is:
The word "church" is used loosely to describe a lot of things.
Sometimes it refers to a building - the one with stained glass windows and a spire.
It can be used to describe a denomination - Presbyterian versus Roman Catholic.
It can be used to describe a local church located in a neighbourhood.
It can be applied more universally to mean all those who are Christians across the world irrespective of what denomination or local congregation they belong to.
Within Christianity there are only two definitions of Church that count - a local congregation of believers (the meeting place is irrelevant); and the universal body of believers across the world.
Secondly the definition of "Christian":
From outside of Christianity, and sometimes from within, Christians are often defined as those who are members of some local church or at least attend church. This definition, although great for those who want to view church and Christianity mere from an administrative perspective, is fraught with difficulty. Countless millions of people are registered as members of some local church who either never attend or only very infrequently, and also adhere to a belief system which is at odds with Biblical Christian belief. Furthermore, there a lot of people, who for various reasons, despite being Christian in belief do not attend church or are not members of any church.
One's beliefs are also important.
Many groups portray themselves as "Christian" - the Mormons and Jehovah's Witnesses are just two prominent examples of thousands of groups - yet their beliefs and doctrines are fundamentally opposed to Biblical Christianity. Many, but not all, these groups also appeal to extra-Biblical authority in the form of books or so-called spoken prophecy.
There are variations in belief and doctrine from within Christianity but certain core doctrine is agreed upon.
In addition knowledge of the doctrine that makes Christianity Christian does not make one a Christian - personally accepting Jesus Christ as one's Lord and Saviour, accepting the work that Jesus Christ did on the cross, and repenting of actual sin in one's life is the fundamental first step in being a Christian.
In depth knowledge of all the doctrine of Christianity is not a prior requirement - certainty of the essential claims that Jesus Christ made about Himself, and the offer of salvation that He extends to us is!
As others have already made mention a Christian is one who has a personal relationship with God, is committed to God and put the priorities of God above their own priorities in life.
Christianity, in fact, is built precisely on the person and work of Jesus Christ. I do not refer to Christianity as a religion either because religiosity is all about ceremony and rules and regulations. Christianity cannot be properly defined apart from being a personal relationship with Jesus Christ.
By the definitions put forward above groups like the Mormons and Jehovah's Witnesses (and lots of others) are not Christian.
Although I believe that there are believers (Christians) within the Roman Catholic Church (RCC) there are several doctrinal beliefs and practices that militate heavily against the average Catholic church-goer coming to a personal relationship with God.
The RCC elevates the Pope and the rest of the clergy to a level of authority that simply cannot be justified Biblically. The doctrine of Papal infallibility essentially makes any proclamation the Pope makes by virtue of His office as binding to the church irrespective of the Biblical position on that particular topic. Ouch!
Furthermore, the RCC places itself firmly as an intermediary between man and God. This is why RCC clergy are called priests. Priests are intermediaries between God and man. However, one of the many consequences of what Jesus Christ accomplished on the cross for us was to remove the need for a priestly intermediate. The concept of a personal relationship with God is a foreign concept if one has to approach God through a third party all the time. In practice Catholics have a relationship with their priest, not God!
I won't go into this in any detail (consult the history books for more details) on this but the enormous manipulation and abuse of power that these two practices within the RCC has resulted in the most appalling consequences over the centuries.
