When you look at systematic theology it is a pretty well defined discipline. There are usually clear boundaries between different subjects in it. Soteriology is different from pneumatology and ecclesiology is different from eschatology. However, when you get down to actually studying the Bible, you recognize these distinctions are a bit arbitrary.
To be fair, to an extent all theology is systematic because we try to organize our thoughts according to whatever conceptual grids we are working with. The Bible does have a structure. It is deeply theological but it is not a theological treatise therefore, what it says is not neatly arranged in the systematic fashion Christians eventually came to do theology. So when we apply these topical divisions to how we read and study the Bible, not only are they a little anachronistic, they are somewhat ill-fitting and awkward. That is because the writers of the Bible did not think or write in terms of how we do systematic theology. Indeed, they couldn’t have used our conceptual frameworks. Something that illustrated this vividly was the debate between N.T. Wright and John Piper.
I did not know about any of them when the debate was in full force but since becoming a fan of Wright’s work I have become reasonably familiar with it. I am certainly not going into the complexities of that long debate on justification but I will talk about the theological angles they both had. Piper came from the mainstream perspective that justification in Romans is about soteriology, that is, a how person is saved, while Wright argued that justification is about ecclesiology, that is how one becomes a member of God’s people. Again there is a lot more to what each said but that is not the issue right now. Going through their various arguments and how they exegeted the relevant texts, I could see Paul was doing both what we call soteriology and ecclesiology. Exactly how that works is a story for another day but Paul was doing other things as well, including what we call eschatology (things regarding the end times.) In fact that there are major issues that are very important to him which are not even defined fields in systematic theology. While Piper to a greater extent and Wright to a lesser extent were both working with later theological categories, Paul himself simply did not know those distinctions. He was working with his own categories and conceptual frameworks. This is not unique to Paul. It is true of the entire Scriptures.
A good example of the arbitrariness of our theological categories that I came across was learning what the Bible means by salvation. As I earlier mentioned, when Paul was talking about justification he was doing what we call soteriology, ecclesiology, eschatology, among other things all at once. In particular, the relationship between soteriology and eschatology is quite strong in Paul and throughout the New Testament. From an exegetical stand point I cannot tell the difference. In 1 Thessalonians 1:10 Paul says Jesus “delivers us”, which is obviously soteriological language, “from the wrath to come” which is eschatological language. Also, in 1 Peter 1:5 it says we are being kept by God’s power for “a salvation ready to be revealed in the last time.”
Many, many other examples could be given about how the theological content of the Bible does not neatly fit our systematic shelves. The different areas in systematics are useful because they provide markers that help us navigate through the dense jungle of the Scriptures. However, they are at best topical approximations of what Scripture is about and not the Scriptures themselves. Systematics is like a political map. Political maps draw useful but artificial boundaries on real unbounded land. If you wish to truly understand the landscape, you have to go on the ground. That is the way the Scriptures work, you have to work with its contours and features.