Until about the end of the 6th century the vocabulary of Old English was more or less entirely Germanic in its origin but with Christianity the Roman priests brought a number of vocabulary items connected with religion. The grammar of the language also changed during this period.
The Middle English period is known for a rapid decay of inflections but it would be wrong to assume that this decay started only after 1150. It is quite likely that this process of change manifesting itself in an increasing loss of inflections started sometime towards the end of the Old English period and that this process was hastened because of the socio-political situations during the Middle English period. It is known, for example, that during the early Old English the word “him” was the dative form and not the accusative form of the third person singular pronoun. Similarly, “me” and “us” in early Old English were the dative forms of the first person pronouns.
It was only towards the end of the Old English period that these dative pronouns acquired an accusative function as well. Much more research has to be done in this area before we can have even a reasonably satisfactory account of the variations of Old English describable along the axis of time.