Christianity is Jesus Christ, nothing more and nothing less. That's why it's called Christianity, and we are known as Christians. The moment we add - or subtract - from that simple fact, we no longer have Christianity.
Christianity is not adhering to a set of beliefs about Jesus or a code of conduct in which we seek to emulate him by following his example. It's not examining his earthly life and teaching as presented in the Gospels and adopting that as a philosophy. It's not observing special days or wearing special clothes. It's not about attending conferences or having a clerical title.
It's very easy to overlay the essence of Christianity with the trappings of what we think should be Christianity; but Jesus plus anything else actually makes Christianity nothing - or less than nothing.
Christianity is Jesus Christ, the Jesus Christ as presented in the Word of God, who lives his own life in and through his Body on earth - the Church. If what we call Christianity or Church lessens that reality, then we cannot truly call ourselves Christians or define ourselves as his Church.