If I had to summarize the sermon on the mount in one sentence, it would be this.
Live in light of the reality that God is your gracious, generous, and powerful father.
I'm not sure I see this in all of the parts of it yet, but it certainly seems to be a theme. Here are some examples:
Why can we rejoice in persecution? Because our faith pleases God. Why should we love our enemies? Because that is what our father does. Why should we pray simply? Because our father already knows what we need. Why should we not be anxious about our life? Because our father cares about us and will take care of us.