Short after the death of emperor Theodosius in 395 a time of unrest started in the Roman Empire. The migration had begun and would bring chaos to entire Europe in a short period. The most important “migration” in Western Europe was the expulsion and withdraw of the Romans from big parts of their imperium, with all consequences.
Other tribes and nations took their place:
The Franks took North Gaul;
The Angles, Saxons, Frisians and Jutes conquered a larg part of Brittain;
The Langobards moved to Northern Italy;
The Alemanni went to the current Southern Germany and Switzerland
and the Thuringians conquered a new habitat to the south of the Harz Mountains.
Important was that the part of the SaxonyPeople (Saksenvolk) that did not take part in the conquer of Brittany, stayed where their ancestors had lived for centuries: The current North-East Netherlands and North-West Germany. That area was never occupied by the Romans, and therefor much less influenced by the Roman culture. The polity and also the religion of the Saxons (Saksen) stayed in big lines as they had always been: Germanic. The migration passed Saksenland!
The Saksenland that had to be converted, was almost the same as the current Niedersachsen + Westfalen + the North-east part of the Netherlands: the area between the rivers IJssel and Elbe.
That is the place where in the early Middle Ages the Old Saxon was spoken.
In the Dutch part of this Saksenland there are now spoken Saxon dialects in the provinces of Groningen, Drenthe, Overijssel, a part of Gelderland (the east end of the Veluwe and the Achterhoek) and a part on the eastern side of Friesland on the border with Drenthe, where the Stellingwerfs is spoken. The saxon dialects in this North-Eastern part of the Netherlands are counted to the Nedersaksische /Niedersachsische / Lower Saxon languages.