There seems to be a lot of confusion on what Christianity is exactly. A Christian understands what it means to "believe in Jesus". That belief is based on various foundations.
First, there are the teachings, from witnesses or their witnesses, in the New Testament. It's important to understand, from a proper education, that these teachings are both explicit (directly stated) and implicit (implied by statements of the witnesses). Furthermore, no single belief exists in a vacuum or stands by itself, but must be understood in context of all teachings.
Second, there are beliefs stated in ancient texts, accepted as orthodox by early Christians, which were not included in the New Testament. These beliefs cannot contradict what's understood in the New Testament, obviously.
Third, there are teachings, and practices (rituals, customs, etc.), which were formalized slightly later, but widely accepted by early Christians with probable origins with the Apostles and Paul as well as the early bishops and presbyters. An example of this is the explicit doctrine of the Trinity, which also happens to be implicit in the New Testament.
A Christian must believe in, and LIVE, these teachings and practices. To deny the New Testament, including its obviously implicit teachings as accepted and spread by the Apostles, Paul and their early successors, means one is not a Christian. The individual cannot pick and choose then claim to be a Christian. So, no, Mormons and Jehovah's Witnesses are not Christians in any meaning.
The differences among Catholics, Eastern Orthodox Christians and the many, many different Protestant groups are another matter entirely.... At the very least they are all in agreement on the core beliefs as stated above.