At issue here is Calvin’s take on the Sermon on the Mount (Matthew 5:7). Calvin had little if any patience with those who tried to argue that this, or the parallel account in Luke 6 (sometimes called the Sermon on the Plain) were two intact speeches given on two separate occasions and recorded in their entirety by the Gospel writers. More likely, these accounts were more a “greatest-hits collection”, gathered together and placed at the beginning of Jesus’ teaching ministry to provide a representative sampling of the main things he taught.
It just so happens that this view happens to align quite nicely with modern historical criticism of the Gospels, what some might call “higher criticism”. What do we do with that? Do we dismiss this view as out of hand because it aligns with what those who, acting from strictly atheistic motivations, seek to deconstruct the Bible and prove its historical unreliability, are doing? Or do we perhaps take a second look at our convictions about how the Bible, as a divinely inspired book of history, ought to work?
The Gospel writers had other priorities than to provide a historically accurate and complete record of everything Jesus ever said and did during his public ministry–at least in the way that we moderns understand historically accurate. Matthew and Luke intended their works to be more doctrinal than historical. They intended them to be used primarily for the purpose of teaching believers about the key teachings and doings of Jesus. For that purpose, it would make sense for them to take the key teachings of Jesus and present them all together, in summary form, at the beginning of Jesus’ teaching ministry.