Some people have a tendency to minimize apostasy. This can be seen, for example, in the King James translators' choice of words in 2 Thessalonians 2:3 where Greek ἀποστασία is translated as "a falling away." While it is certainly possible to dwindle in unbelief and go slowly, surely, incrementally, and inevitably to hell, one can also do so dramatically, drastically, and definitively.
A good gauge for what the term would have mean to Paul and his converts from Judaism can be found in the Septuagint version of 2 Chronicles 21. The Septuagint was the Jewish translation of the Hebrew Bible into Greek. For most Mediterranean Jews of Jesus's day, the Septuagint was the holy scriptures. In Chronicles it reads that "In these days Edom apostatized (ἀπέστη) from Judah" (2 Chronicles 21:8). Here it is clear that Edom did not just "fall away;" Edom revolted or rebelled and declared its independence. The Hebrew verb used here, pāšaꜥ, simply means to rebel. Thus "Edom apostatized from/rebelled against Judah until that day" (2 Chronicles 21:10).
Apostasy is thus an open rebellion, a revolt, a mutiny. While we use the English term in a religious context, the Greek term originally had a military or political context.
Edom doubtlessly depicted their actions as merely a reorganization and reorientation that would align their territory among the nations in a way that would better serve its goals, as a new beginning and new direction for Edom, but the Chronicler was more blunt: Edom was in apostasy.