I suppose that in many ways it can truthfully be said that the greatest need of men and women in this world is the need of what is called a quiet heart, a heart at leisure from itself.
Is that not, in the last analysis, the thing for which we are all looking? You can if you like call it peace; that means exactly the same thing, peace of mind and peace of heart, tranquillity. We are all restless; we are all disturbed. There is unhappiness in us, and it is produced by many different causes.
One thing that causes all our hearts to be restless and disturbed, one thing that robs everybody of peace, is the thought of death. This is a great and certain fact; in the words of the woman of Tekoah, “For we must needs die, and are as water spilt on the ground, which cannot be gathered up again” (2 Samuel 14:14). That is a most disturbing, a most troubling thought. The author of the Epistle to the Hebrews says that until we become Christians, we are all in lifelong “bondage . . . through fear of death” (Hebrews 2:15). Shakespeare, who knew the human heart, gives these words to Hamlet: “The dread of something after death, the undiscovered country, from whose bourn no traveller returns.”
“Conscience,” he adds, “doth make cowards of us all.” Yes, we do this and that, but thought of that “undiscovered country” upsets everything. That is the trouble and that is the cause of the restless, unquiet heart.
Martyn Lloyd-Jones, Let Not Your Heart Be Troubled (Kindle Edition)