Why did Paul say Sinai was in Arabia?

Answer:

You are correct that modern-day boundaries are not necessarily what was used in the past. For example, what we now call "Asia Minor" was called "Asia" during the time of Paul (Acts 16:6). What we currently call "Asia" had a different name. Yet, if you are unaware of the shift in names, you might think that Paul had preached in Russia or China.

"For this Hagar is Mount Sinai in Arabia, and corresponds to Jerusalem which now is, and is in bondage with her children" (Galatians 4:25).

Mount Sinai was counted in the territory of Arabia in Paul's day. Since both the Arabian Peninsula and the Sinai Peninsula are both desert regions, it wouldn't be difficult to understand if past cultures considered them to be basically the same region. Graham Davies, in Wilderness (1979), stated, "In Herodotus ca. 450 B.C., not only was the Sinai Peninsula considered part of Arabia but, surprisingly, so was all of the eastern desert half of what we now call Egypt on the continent of Africa. In Greek and Roman Empire days, the bulk of the Sinai Peninsula was left to the Nabateans as "Arabia Petrea," until their conquest by the Romans in 106 A.D. The southern and central Sinai were then merged into the new Province of Arabia. Even in modern times, Wilhelm Gesenius listed both Mt. Sinai and the Sinai Peninsula as part of "Arabia," in his famous 1834 Hebrew Lexicon."

"The Galatians 4:25 reference might indeed support the view that Mount Sinai was in Saudi Arabia if the Apostle Paul was looking at a 1990 Rand McNally Atlas. However, it would not be true if he was looking at a First Century AD Roman road map. Although no actual maps of Roman Arabia exist from this period, we do possess the accounts of the contemporary travelers such as Strabo, a Greek from Pontus (64 BC to ca. AD 25). He describes the borders of Arabia as having its eastern border at the Persian Gulf and its western border at the East Side of the Nile River. This means that Strabo understood the entire Arabian Peninsula and the Sinai Peninsula to be included in First Century Arabia (Geography 16:4:2; 17:1:21,24-26,30,31; LCL VII: 309; VIII: 71-79, 85-87)" [Is Mount Sinai in Saudi Arabia?, Associates for Biblical Research].

"Moreover, in the mid-third century BC, 72 Jewish scholars translated the Hebrew Bible into Greek (known as the Septuagint) and followed the contemporary use of the word “Arabia” when they referred to Goshen as “Goshen of Arabia” (Gen. 45:10; 46:34). While Goshen is clearly part of Egypt (Gen. 37:6, 27; Ex. 9:26), the translator imposed the third century BC geographical reality on their translation" [Is Mount Sinai in Saudi Arabia?, Associates for Biblical Research].

What we know of the location of Sinai:

1. It was an eleven-day journey from Mt. Sinai to Kadesh-Barnea on the southern edge of Canaan (Deuteronomy 1:2).
2. It wasn't in the territory of Midian. "Then Moses let his father-in-law depart, and he went his way to his own land" (Exodus 18:37). "Now Moses said to Hobab the son of Reuel the Midianite, Moses' father-in-law, "We are setting out for the place of which the LORD said, 'I will give it to you.' Come with us, and we will treat you well; for the LORD has promised good things to Israel." And he said to him, "I will not go, but I will depart to my own land and to my relatives" (Numbers 10:29-30). Midian was located east of the Gulf of Aqaba in northwest Arabia.
3. Josephus, in Against Apion, quoting Apion says, "Moses went up to a mountain that lay between Egypt and Arabia, which was called Sinai...."