Hello! It's been my dream for a long time now to essentially become a master in an area of study (very vague, I know). But I wish to be consider an expert on something. But my biggest problem is that my interests varied widely and I feel as though I can only have basic, surface level knowledge of a subject. I am very jealous of historians who have seem to have found their passion and their niche, and I want to discover my own! Are there any tips on how best to go about this? If it helps, my interests includes:
Finding a passion is easy, getting paid for it is hard.
But seriously, what stage are you at, education wise? I think that will tailor some of your/our advice here.
History of Christianity requires languages. It requires a ton of them. So get cracking now. Depending upon period, you're looking at Latin and/or Greek, almost certainly German, possible French and/or Italian. Sometimes Hebrew. And these are just for starters.
I would work on getting a good overview knowledge of Christian History, then specialise. For example, I took undergraduate classes that covered the range of Christian history, then did graduate work in Ancient History specifically, and my doctoral dissertation focuses on a very thin slice of the 4th century.
On 4 - doing cross-disciplinary stuff is hard because it's hard to be adequately competent in multiple areas. Unless that is your one and only area. So either you skill-up in one speciality and then turn one of the others into you secondary suite of history powers, or you try and do everything all at once but risk the problem of not being good enough at any of the three individually to make reasonable contributions that don't get dismissed for lacking depth and rigour.
As has been stated, the level of education you are at is essential to this question. Generally, achieving mastery in a subject requires years of reading, a very firm grasp of the relevant literature in the most relevant languages, and a firm knowledge of the relevant primary sources. Some people achieve this by writing a PhD (your first original contribution to the field usually), some through self-study. I've been educating myself and been educated in my area of expertise for 10 years now, am now at PhD level and I wouldn't consider myself a master in my area of study.
As for narrowing down interests, one thing that is important here is that all these are very long histories (2000 years of Christianity, 1300 years of history of Islam, 200+ history of Socialism) and one thing that can help out here is finding a case study to read about. That case study can originate from what resources you have available (speak German? Potential correlation of the success of Communism in Catholic vs. Protestant areas of Germany can be a subject! Speak Arabic? Pan-Arab socialism and its relationship to Islam here you come! etc.). In my experience mastery of an area of expertise is caused by going deep into it to acquire broad knowledge.
The shortest path to becoming an expert is to become the authority on a subject in your backyard - know the history of something near because the research will be much easier.
That said, sometimes passion leads elsewhere. I suggest you focus on something small - an area or an ethnic group that has experienced Islam, Christianity, and contact/influence from the Soviet Union, China, and/or Cuba: the Kurdish area with its Christian (and other) minorities; one of the small Arabic states; a place in Sub-Saharan Africa; a group of people in Southeast Asia. Spend several years reading everything and learning to read the language(s) needed and start writing articles on the subject.
If you are self-taught, you will have a hard time understanding the big picture, comprehending how other experts view the region and subject as a whole. You'll need to read the larger histories of the region and explore the internet for opportunities to exchange ideas so you can acquire as much of that larger perspective as possible.
If you have the fire in your belly and you pick a subject and area that fuels your passion, you will become an expert. It will be hard work and require diligence, but it can be done.