Why do we assume that in his sermon on the mount Jesus addresses the whole church throughout the ages? Much of the teaching has to do with what it means to fulfil the Law of Moses, which Jesus categorically says he has not come to abolish—at least, not until heaven and earth pass away (5:17). The unrighteous are threatened with the same fate (corpses thrown into Gehenna) that Jeremiah prophesied for Jews in Jerusalem ahead of the Babylonian invasion. Jesus borrows metaphors from the prophets (two roads and a house swept away by a storm) to describe the dilemma faced by first century Israel under Roman occupation. He speaks as a prophet to Israel about Israel.
So why are we so eager to read Matthew 5-7 as a compendium of classic Christian wisdom or a manifesto of radical Christian ethics? Presumably because we find in it such powerful and enduring sentiments as “Blessed are the peacemakers”, “You are the light of the world”, “everyone who looks at a woman with lustful intent, etc.”, “Love your enemies”, “You cannot serve God and money”, “seek first the kingdom of God”, and so on.