“One of the major themes throughout Luke-Acts is the universalization of the Gospel ~ that its for all people from the last, least, and lost to the first, most, and found. This is a theme Luke found in his favorite prophetic book, Isaiah, and it is highlighted both early and often in Luke-Acts (cf., e.g., Luke 3:6, “all flesh shall see the salvation of God,” to Isa. 52:10). The story of the Ethiopian eunuch may be said to be Exhibit A of this promise being fulfilled, not least because the actual text of Isa. 52:10 says all the ends of the earth shall see God’s salvation, and that is precisely where the Ethiopian comes from as things were viewed in Luke’s day.
It is easy to forget that the ancient world was not simply divided into Jews and Greeks and Romans and the various peoples who were their clients or relatives or were subjected within the immediate bounds of the Empire, those whom Greco-Romans would have seen as living at the ends of the earth, including some of those whom the Greeks called Barbarians, by which meant non-Greek-speaking peoples (cf. Acts 28:2). A reasonable case can be made for seeing this narrative as being about the reaching of those from the parts of Africa that were at or beyond the borders of the Empire, those that were at the ends of the earth.”
(Witherington, The Acts of the Apostles – A Socio Rhetorical Commentary, p.293)