“Where your fear is, there is your task.”

Order of the Day (2 June 1944), a message to troops before the Normandy landings http://www.3ad.com/history/wwll/feature.pages/d.day.letters.htm, reported in Franklin Watts, Voices of History (1945), p. 260
1940s
Context: Soldiers, Sailors and Airmen of the Allied Expeditionary Force!
You are about to embark upon the Great Crusade, toward which we have striven these many months. The eyes of the world are upon you. The hopes and prayers of liberty-loving people everywhere march with you. In company with our brave Allies and brothers-in-arms on other Fronts, you will bring about the destruction of the German war machine, the elimination of Nazi tyranny over the oppressed peoples of Europe, and security for ourselves in a free world.
Your task will not be an easy one. Your enemy is well trained, well equipped and battle hardened. He will fight savagely.
But this is the year 1944! Much has happened since the Nazi triumphs of 1940-41. The United Nations have inflicted upon the Germans great defeats, in open battle, man-to-man. Our air offensive has seriously reduced their strength in the air and their capacity to wage war on the ground. Our Home Fronts have given us an overwhelming superiority in weapons and munitions of war, and placed at our disposal great reserves of trained fighting men. The tide has turned! The free men of the world are marching together to Victory!
I have full confidence in your courage and devotion to duty and skill in battle.
We will accept nothing less than full Victory! Good luck! And let us beseech the blessing of Almighty God upon this great and noble undertaking.

Citadelle or The Wisdom of the Sands (1948)

"Going up to Jerusalem", Twenty Sermons (1886), p. 330.
Context: O, do not pray for easy lives. Pray to be stronger men! Do not pray for tasks equal to your powers. Pray for powers equal to your tasks! Then the doing of your work shall be no miracle. But you shall be a miracle. Every day you shall wonder at yourself, at the richness of life which has come to you by the grace of God.

R.U.R. (1920)

Narrator
A Child is Born (1942)
Context: I'm your narrator. It's my task to say
Just where and how things happen in our play,
Set the bare stage with words instead of props
And keep on talking till the curtain drops. …
It's an old task — old as the human heart,
Old as those bygone players and their art
Who, in old days when faith was nearer earth,
Played out the mystery of Jesus' birth
In hall or village green or market square
For all who chose to come and see them there,
And, if they knew that King Herod, in his crown,
Was really Wat, the cobbler of the town,
And Tom, the fool, played Abraham the Wise,
They did not care. They saw with other eyes.
The story was their own — not far away,
As real as if it happened yesterday,
Full of all awe and wonder yet so near,
A marvelous thing that could have happened here
In their own town — a star that could have blazed
On their own shepherds, leaving them amazed,
Frightened and questioning and following still
To the bare stable — and the miracle.

Book I, line 98
De Arte Poetica (1527)

Commonly attributed to Twain in computer contexts and post-2000 inspirational books — the first sentence has also been attributed to Agatha Christie and Sally Berger.
Misattributed

Source: The Chronicles of Prydain (1964–1968), Book V : The High King (1968), Chapter 21
Context: “Dyrnwyn is yours,” Gwydion said, “as it was meant to be.”
“Yet Arawn is slain,” Taran replied. “Evil is conquered and the blade’s work done.”
“Evil conquered?” said Gwydion. “You have learned much, but learn this last and hardest of lessons. You have conquered only the enchantments of evil. That was the easiest of your tasks, only a beginning, not an ending. Do you believe evil itself so quickly overcome? Not so long as men still hate and slay each other, when greed and anger goad them. Against these even a flaming sword cannot prevail, but only that portion of good in all men’s hearts whose flame can never be quenched.