Recently I was writing up some material introducing 'Patristic Greek' which is not helped by the fact that there is no such entity. This lead me to reflect a little on idiolects and 'languages'.
Many years ago when I was studying for my first degree I took philosophy of language, and we discussed the idea that there aren't any languages. Or, to put it another way, everyone has their own language. We call this an idiolect. Really the division of a collection of signifiers and signifieds into discrete units we call languages is as arbitrary as the relation between signifiers and signifieds in the first place. This is why there is no limit to English. You're always learning more English, because no-one's idiolect is entirely co-extensive with every element of communication found among all speakers of 'English' who identify those elements as 'English', and English is quite promiscuous in that we readily identify incorporated elements of other languages as 'English' in a way that other languages do not so easily indigenise 'foreign' expressions.
To demarcate something as a 'language', I would propose two elements: cohesion and complexity. The system must be cohesive enough to be readily identified by users as a single 'entity', even if the edges of that entity are frayed and difficult to draw. Secondly, the system must have some minimal complexity requirements. It needs to be able to achieve the regular goals of communication. This, of course, is why languages tend to have so many similar features - we want them to achieve certain goals. Of course, they do that in differing ways, which is why they are different! and some features of certain languages are just absent in others - to replicate those features you have to 'reroute' through other features to reach the same ends.
When you come to read Patristic Greek, you are dealing with a collection of idiolects - an array of writers using their own systems of communication that broadly but not necessarily overlap. In some instances those idiolects show the 'interference' of other discrete-ish systems, i.e. other languages. IN the end each one is entirely individual. The construct of Patristic Greek is merely taking the amalgam of all these idiolects, and subtracting it out of the much larger set of 'Classical and Hellenistic Greek'; which of course is itself a somewhat arbitrary construct.
What's the best way to learn Patristic Greek? I would tell people to lay a good foundation in Classical Greek; if you're going to pick a grammar, pick one that is Classically orientated rather than Koine. Read some prose and oratory. Then become acquainted with the New Testament, move on to the Apostolic Fathers, and from there launch into LXX and the Fathers.