The series itself is very old. I remember hearing about it when I was a child. I imagine that it still uses the same gameplay mechanics as Wolf Kanno mentioned, so it's likely prone to a higher difficulty and an unforgiving nature compared to most games.

You could probably play some of the older games by an emulator if you're interested in getting to know the series. I know that it's difficult as there are many games in that series that were never released here because they deal with popular religious themes and deities; therefore, you have to figure out which ones were released here, which ones have fan translations, and so on.