Reb Tzaddok HaCohen writes, "Just as one must believe in Hashem, so must one believe in himself. He must believe that Hashem is interested in him, that he isn't just anybody… One must believe his soul comes from Hashem Yisbarach, the origin of all life, and Hashem derives pleasure when he does Hashem's will. This is the meaning of the verse (Exodus 14:31), 'They believed in Hashem and in Moshe His servant.' "Moshe" [in this verse] every Jew… They believed that Hashem wants them."
We say in the Yom Kippur prayer, Hashem desires the praises that come from limited human beings. He wants our service. It doesn’t seem rational, and therefore people have trouble believing it. But that is the reality. This is the emunah (faith) that the nation attained at the Sea of Reeds, as it states, they believed in Hashem, and they believed in Moshe, they believed in themselves, too. This essential emunah didn’t come to them immediately, as it states (Psalms 106:7), "They rebelled on the sea, at the Sea of Reeds." Our Sages (Eiruchin 15.) explain, "The Jews had weak emunah… They said, 'Just as we are leaving the sea from one side, the Egyptians are leaving the sea from another side…" What was the nature of their rebellion, and why did they think that the Egyptians were coming out alive? After all the miracles Hashem performed for them, they should have believed in Hashem. But the explanation is, they definitely believed in Hashem. The problem was that they didn’t believe in themselves.
My grandfather, Rebbe Moshe Mordechai explains that they couldn’t imagine that they were so precious to Hashem, that Hashem would save only them and not the Egyptians. Therefore, they said, "Just as we are being saved, perhaps the Egyptians are being saved too…" So apparently, at first, when the sea split, the Jewish nation was still with weak emunah. They didn’t believe in their own greatness. But eventually, their emunah grew until they believed in their own greatness, and they sang Oz Yashir. It is important to discuss these matters, because the yetzer hara (Evil Inclination) tells people they are not important, and that Hashem doesn’t care whether they do a mitzvah or commit a sin, G-d forbid. It is extremely important to believe that Hashem wants your prayers, your service, and rejoices immensely with every success and has distress with every sin. A person should tell the yetzer hara, "It isn't as you say. I am great. I am holy. Hashem wants my service."
This is what Yosef HaTzaddik said when he was tested by Photiphar's wife (Genesis 39:9), "No one in this house is greater than me." Tzaddikim explain that Yosef was saying that he is a great tzaddik and totally distant from sin. With this conviction, he was able to pass the test and avoid sin. The Orach LaChaim (Zlotchev) zt'l explains every Jew has many strengths, talents, and abilities. Every Jew has the ability to make revolutions and radical changes. They can turn everything over and accomplish great matters – both in the spiritual realm and also in regards to material accomplishments. They can accomplish so much. So what holds them back? They don’t believe in themselves. They don’t realize that they have these strengths