“It is time for us to realize that we're too great a nation to limit ourselves to small dreams.”

The third and fourth sentences are a paraphrase of a sentence by G. K. Chesterton: "I do not believe in a fate that falls on men however they act; but I do believe in a fate that falls on them unless they act." Generally Speaking, "On Holland' (1928).
1980s, First term of office (1981–1985), First Inaugural address (1981)
Context: It is time for us to realize that we're too great a nation to limit ourselves to small dreams. We're not, as some would have us believe, doomed to an inevitable decline. I do not believe in a fate that will fall on us no matter what we do. I do believe in a fate that will fall on us if we do nothing. So, with all the creative energy at our command, let us begin an era of national renewal. Let us renew our determination, our courage, and our strength. And let us renew our faith and our hope. We have every right to dream heroic dreams. Those who say that we're in a time when there are no heroes, they just don't know where to look.