We must believe that everything is for the good and that everything is from Hashem. The belief itself has the potential to turn everything around and cause the good to occur. Sifsei Tzaddik writes, "When one believes in nature, matters generally don't change, but when one believes in Hashem, things can change in a moment."
The Midrash says that when Sarah was taken captive in Mitzrayim, she lay on her face all night and davened. She said, "Ribono Shel Olam! Avram was saved with Your promise [because Hashem promised to protect Avram], and I was saved with emunah [I believe Hashem will protect me]. Yet, Avram is now outside [and away from danger], and I am in the pot [in grave danger]."
The Divrei Yisrael zt'l writes that it seems from this Midrash that Sarah's main point was that since she had emunah, she should have been saved even more than Avraham, who had received a promise. But this is difficult to understand. Avraham had a promise, a guarantee that Hashem would protect him, while Sarah hadn’t. This should explain why Avraham was safe while Sarah wasn't.
The Divrei Yisrael gives a beautiful answer. He says that when one has emunah that Hashem will help, it will certainly be so. He is even more guaranteed than if one receives a promise.
Let us elaborate so we will understand this great lesson:
How would you feel if a great tzaddik told you that everything would be good, you will be protected and wealthy? How would you feel if Eliyahu HaNavi gave you this promise? How would you feel if Hashem, Himself, promised this to you? Everyone would be thrilled to hear that. However, to believe that Hashem will help is even greater. When you have emunah that Hashem will help, it will undoubtedly transpire.
This is what Sarah was saying. Avraham was safe and out of danger, and he received Hashem's word. I have more than that. I have emunah in Hashem that He will save me. Emunah is the highest level and the greatest guarantee. So why is my life in danger now?"
Rabbeinu b'Chayei writes, כאילו 'בה הבוטח ה"הקב הבטיחו, "When one has bitachon, it is like Hashem promised him." When he has bitachon, he can feel confident that Hashem will help him. He can view it as though Hashem told him everything will be good. And as the Divrei Yisrael reveals to us, he can be even more confident, because emunah and bitachon are the strongest guarantees that everything will be good.
