“Mordre wol out, that se we day by day.”

"The Gods" (1876) as published in The Gods and Other Lectures (1879).
Context: Day by day, religious conceptions grow less and less intense. Day by day, the old spirit dies out of book and creed. The burning enthusiasm, the quenchless zeal of the early church have gone, never, never to return. The ceremonies remain, but the ancient faith is fading out of the human heart. The worn-out arguments fail to convince, and denunciations that once blanched the faces of a race, excite in us only derision and disgust. As time rolls on, the miracles grow mean and small, and the evidences our fathers thought conclusive utterly fail to satisfy us.

Source: When Women Were Birds: Fifty-four Variations on Voice

This appears on the opening placard of the film The Equalizer, attributing it to Twain, but there is no evidence that Twain wrote it. A precursor is found in Taylor Hartman's self-help book The Character Code (first published 1991), where it is not attributed to Twain: "The three most significant days in your life are: 1. The day you were born. 2. The day you find out why you were born. 3. The day you discover how to contribute the gift you were born to give" ( Google Books link https://books.google.com/books?id=gIKCxWxNmeMC&pg=PA147&dq=%22day+you+find+out+why%22&hl=en&sa=X&ved=0ahUKEwijrJzc84vLAhUJzGMKHajvADEQ6AEIIjAB#v=onepage&q=%22day%20you%20find%20out%20why%22&f=false)
Disputed

From his 1971 World Series MVP acceptance speech, discussing his sometimes strained relationship with manager Danny Murtaugh, as quoted in "Pittsburgh's Clemente Honored"
Baseball-related, <big><big>1970s</big></big>, <big>1971</big>

Source: Beyond This Horizon (1948; originally serialized in 1942), Chapter 14, “—and beat him when he sneezes”, p. 134

And on that day, our nation shall fulfill its creed — and that fulfillment shall enrich us all.
What the Future Holds (1984)