One of the things Jesus was doing in the Sermon on the Mount was countering the teachings of the Pharisees. The Pharisees had created rules and traditions that watered down the Law in order to make it "doable" for them. (Jesus told them, "you nullify/make void the Word of God by your tradition" - Matt 15:6, Mark 7:13). In the sermon, Jesus made clear what the Law really said and meant, as opposed to the spin the Pharisees had put on it. Let me emphasize something: in the sermon, Jesus was teaching the law, bringing back its true meaning and intent (which effectively showed just how impossible it is to keep it).
Ironically, what the church has done today is they have watered down the Sermon on the Mount, putting a spin on the various things Jesus said in it, in an attempt to make His hard words "doable" or to make the sermon into a "Christian" teaching. Most likely unknowingly, what they have done is diminished, and thereby nullified, what Jesus was actually saying. In trying to make this difficult law sermon into a teaching on practical Christian living, they are oblivious to how they have made void the Word of God.
I don't say this to complain about the church, but to make the larger point that in order to truly understand the gospel, we have to recognize the things that are taught in the church today as the gospel that really aren't the gospel.