Consider Mother Teresa, the epitome of Christian service in the 20th century. Her compassion for the poor was tremendous, winning the Nobel Peace Prize in 1979. Nevertheless, belief was rarely easy or even certain. Check out this excerpt from her letters, encapsulated in Mother Teresa: Come Be My Light.
Now Father—since  or 50 this terrible sense of loss—this untold darkness—this loneliness—this continual longing for God—which gives me that pain deep down in my heart.—Darkness is such that I really do not see—neither with my mind nor with my reason.—The place of God in my soul is blank.—There is no God in me.—When the pain of longing is so great—I just long & long for God—and then it is that I feel—He does not want me—He is not there.—Heaven—souls—why these are just words—which mean nothing to me.—My very life seems so contradictory. I help souls—to go where?—Why all this? Where is the soul in my very being? God does not want me.—Sometimes—I just hear my own heart cry out—“My God” and nothing else comes.—The torture and pain I can’t explain.—From my childhood I have had a most tender love for Jesus in the Blessed Sacrament—but this too has gone.—I feel nothing before Jesus—and yet I would not miss Holy Com. [Communion] for anything. You see, Father, the contradiction in my life. I long for God—I want to love Him—to love Him much—to live only for Love of Him—to love only—and yet there is but pain—longing and no love.—Years back—about 17 year now5—I wanted to give God something very beautiful.—I bound myself under pain of Mortal Sin not to refuse Him anything.—Since then I have kept this promise—and when sometimes the darkness is very dark—& I am on the verge of saying “No to God” the thought of that promise pulls me up. I want only God in my life—“The work” is really and solely His.—He asked—He told me what to do—He guided every step—directs every movement I take—puts the words in my mouth makes me teach the Sisters the way.—All that & everything in me is He.—This is why when the world praises me—it really does not touch—not even the surface—of my soul. About the Work I am convinced it is all He. Before I could spend hours before Our Lord—loving Him—talking to Him—and now—not even meditation goes properly—nothing but “My God”—even that sometimes does not come.—Yet deep down somewhere in my heart that longing for God keeps breaking through the darkness. When outside—in the work—or meeting people—there is a presence—of somebody living6 very close—in very me.—I don’t know what this is—but very often even every day—that love in me for God grows more real.—I find myself telling Jesus unconsciously most strange tokens of love.— Father, I have opened my heart to you.—Teach me to love God—teach me to love Him much. I am not learned—I don’t know many things about the things of God.—I want to love God as and what He is to me, “My Father.” Very often I long to make use of the food I give my Sisters—but I can never do it—the same for spiritual books. All these things were so natural to me before—until Our Lord came fully in my life—I loved God with all the powers of a child’s Heart. He was the centre of everything I did & said.—Now Father—it [is] so dark, so different and yet my everything is His—in spite of Him not wanting me, not caring as if for me. When the work started—I knew what it will all mean.— But with my whole heart I accepted then everything.—Only one prayer I made—to give me grace to give saints to the Church. My Sisters, Father, are the gift of God to me, they are sacred to me—each one of them. That is why I love them—more than I love myself.—They are a very great part of my life. My heart & soul & body belongs only to God—that He has thrown away as unwanted the child of His Love.—And to this Father I have made that resolution in this retreat—
To be at His disposal. Let Him do with me whatever He wants as He wants for as long as He wants. If my darkness is light to some soul—even if it be nothing to nobody—I am perfectly happy—to be God’s flower of the field.
Mother Teresa. Mother Teresa: Come Be My Light: The Private Writings of the Saint of Calcutta (pp. 210-212). The Crown Publishing Group. Kindle Edition.