Is Christianity a ‘package deal’, in the sense that one must accept a specific group of beliefs in order to be a Christian? This might seem plausible, as certain denominations self-identify largely on (what they believe is) orthodoxy – certain beliefs (which lead to certain practices) which distinguish them from other denominations. Similarly, there is a history of schism and excommunication based on heterodoxy – people holding beliefs that don’t fit within a certain belief framework of a given denomination.
1. The first thing to note is that, within a given Christian denomination, there is usually a large amount of variety in personal beliefs, even given certain supposed orthodoxy. This is true within Catholicism, and often more so in other denominations. This comes in terms of heterodox beliefs and practices that are within the ‘official’ views of a given denomination, and then in terms of heterodox beliefs that, even though not in line with official views, are held by members of that denomination.
Of course, when it comes to a social institution (which is the essence of what a church is) there must be coordination if there are to be certain practices (such as, say, holding a certain kind of Mass), and this requires a certain amount of overlap in belief. As a side note, this isn’t unique to Christianity (or religion in general) – a requirement for certain amount of overlap of certain beliefs and practices among members is essential for all social institutions.
2. The second thing to note is that given denominations, like basically any social institutions, change their official beliefs and practices over time. This is usually motivated by 1. or 3.-4.
3. The third thing to note is that one can of course start a new church, with a new denomination, with different ‘official’ beliefs.
4. Fourth, as has been increasingly popular in certain places over the last while, one can start a new church and make it ‘non-denominational’. Similarly, you have theological trends like ‘Mere Christianity’, which focus on encompassing heterodoxy.
5. Fifth, one can simply not be a part of an ‘official’ church, building one’s Christianity on a personal or familial level. Christianity at these levels is perhaps the most important form of Christianity.
In summary, although there is something to the idea of Christian denominations being ‘package deals’, there is variety and practically constant change within denominations, new denominations that start, and, of course, being in a denomination isn’t necessary for one to be or become Christian.
So what is required for one to be a Christian? Probably just seeking to understand the Christ, i.e., believing that there is a living Christ (historically rooted in the life and teachings of Jesus of Nazareth, at the least) and that this is a potential (and in certain respects unique) path (‘way’) to theosis.