It seems to me that there has to be at least one group of people who stayed abreast of Abrahamic prophecy and were able to transition from old to new as the religious traditions evolved and spread.
Three regions would really match up with these transitions.
Levant/Egypt
Horn of Africa
the Maghreb
All these regions share a common denominator in that they are all inhabited by Afroasiatic people's and share many common cultural and ethnic features to begin with, as well as strong ties to the middle east for the Horn of Africa and Maghreb region, and a shared cultural milleiu throughout these regions.
Because of these factors, the Abrahamic religions were introduced to these areas at very early dates - or were founded in them, as is for the Levant.
To begin with, Judaism, being the first Abrahamic religion, was probably already well established in the Maghreb and in the Horn of Africa around 100 BC. Jews coexisted and intermarried heavily with the pagan Cushitic and Semitic Horners, as well as pagan Berbers, and were heavily assimilated into their societies. Jews also influenced the groups they lived amongst in these areas both religiously and culturally, but mostly religously, a good example being a mixed Cushitic pagan-Judaic religion of the Ageu highlanders.
Christianity also had its first followers in these regions, and Christianity was present in Egypt from the very beginning, spreading westward to the pagan and also the partially Judaized Berber tribes. It also spread into the Horn, and Christian communities were well established there by 4th century AD, with King Ezana's declaration of Axum as an Christian state. It was also present amongst the pagan Somali clans in northern Somalia, who also contained Hebraic elements within their society.
As for Islam, it's first presence outside Arabia was in the Horn of Africa, with the first mosque outside of Medina being built in the northern Somali port city of Zeila, as well as the Prophet Muhammad sw own daughter, son in law, and other companions fleeing to the court of the Abyssinian monarch. Soon after the Prophets sw death, Islamic armies went into Egypt and the Levant, and westwards into the Maghreb. All these regions were the first areas were Islam was introduced and were early heartlands of the religion, and Islam replaced a lot of both the Judaic and Christian elements in all these regions by converting the Judaized and Christian Berber tribes, the partially Christian and Hebraic-influenced mostly pagan Somali clans and Christian and Judaic Ethiosemites, and a lot of the Christian and Jewish Egyptians and Levantines.