Long ago, at many times and in many ways, God spoke to our fathers by the prophets,
God formerly, etc. This beginning is for the purpose of commending the doctrine taught by Christ; for it shows that we ought not only reverently to receive it, but also to be satisfied with it alone. That we may understand this more clearly, we must observe the contrast between each of the clauses.
God spoke to the fathers through the prophets in scattered pieces across different ages and ways, first to Adam about the Seed of the woman, then to Abraham, Jacob, David, each time clarifying more of the Redeemer's coming. Both dispensations were good, but the gospel is far superior in its manner and fullness.
AI summary
Commenting on Hebrews 1:1-3
God, who at sundry times and in divers manners,.... The apostle begins the epistle with an account of the revelation God has made of his mind and will in former times: the author of this revelation is God, not essentially, but personally considered, even God the Father, as distinguished from his Son in the next verse; for the revelation under the Old Testament is divine...