Below is Martin Luther's definition of believing in God's grace.
Here you can see what it means to believe. It may indeed seem an easy matter, but it is, in fact, a high and great art.
In the face of difficulty
When you feel your sin, when your bad conscience smites you, or when persecution comes, then ask yourself whether you really believe.
At such times one is wont to run to saints and helpers in cloisters and in the desert for succor and relief, crying: “O my dear man, intercede for me! O dear saint, help me! O let me live! I promise to become pious and to do many good works.”
That is how a terrified conscience speaks. But tell me, where is faith?
Trust the Father's Jesus-won promises
If you believe in the words of Christ, “None of them is lost whom thou hast given me” (John 17:12), then, as a Christian, you must say: “I acknowledge no saint here. I am a poor sinner deserving of death, but in defiance of sin and death I cling to thee, and I will not let thee go. I have taken hold of thee, dear Lord Christ. Thou art my life, and this is the Father’s will, that all who adhere to thee have eternal life and be raised from the dead. In the meantime let my fate be what it will. I may be beheaded or burned at the stake.”
No other life—whether it be called the monastic life or the life of St. Augustine or of St. John the Baptist—will arm a person for victory. Only faith in Christ can do so.