This is שבת שירה, and after the splitting of the sea, the Torah says that the people believed in Moshe:
The question is, what did they believe? They believed him when he first came to them:
And they believed him at מעמד הר סיני:
We understand the difference between the latter two “אמונות”. They represent two levels of faith in Moshe’s prophecy. When he first came to Egypt, he did אותות to convince the people that ה׳ had sent him:
But faith in a נביא only applies to that generation (a נביא cannot establish a law that lasts, but only a הוראת שעה). Moshe, at מעמד הר סיני, was stronger than that.
But what was this apparent intermediate stage, of ויאמינו בה׳ ובמשה עבדו? Rav Hutner summarizes the problem:
Rav Hutner proposes that this intermediate stage is related to another role of Moshe, one that does not appear to be related to אמונה at all.
What does the בית דין הגדול mean, in the context of קידוש החודש? My son-in-law, Rabbi Aron Rubin, gave a shiur last week about that, and said that Rav Soloveitchik explained that the Sanhedrin has two roles. One, to be the supreme court of the nation. The other is to represent the will of the nation, and it is in this aspect that they are acting in קידוש החודש.
So Moshe achieved the status of “sanhedrin” when he was given the laws of קידוש החודש; he was the representative of the nation as a whole. But that was well before קריעת ים סוף, and not dependent on ויאמינו…במשה at all.
The אמונה that בני ישראל had at קריעת ים סוף was in במשה עבדו. What is the significance of being an עבד?
So ויאמינו בה׳ ובמשה עבדו meant that בני ישראל believed that Moshe could speak to ה׳ at any time, and ה׳ would listen, and that Moshe represented them, the people as a whole. He stood in the place of the בית דין הגדול, and the בית דין הגדול stood in the place of the עם.
ים סוף was the first time the people asked Moshe to intercede with ה׳ for them. When the sea split, they understood that this intercession was possible.
When Moshe did the signs, back in פרשת שמות, בני ישראל believed that G-d spoke to him. At ים סוף, they believed that he could (and would) speak to G-d on their behalf. Both were necessary for Moshe to achieve that next level, of bringing the Torah to the people.