וַיַּ֨רְא יִשְׂרָאֵ֜ל אֶת־הַיָּ֣ד הַגְּדֹלָ֗ה אֲשֶׁ֨ר עָשָׂ֤ה יְהוָה֙ בְּמִצְרַ֔יִם וַיִּֽירְא֥וּ הָעָ֖ם אֶת־יְהוָ֑ה וַיַּֽאֲמִ֙ינוּ֙ בַּֽיהוָ֔ה וּבְמֹשֶׁ֖ה עַבְדּֽוֹ׃ (פ)
Da sah Israel die gewaltige Hand, welche der Herr betätigte an Ägypten, und das Volk fürchtete den Herrn und glaubte an den Herrn und seinen Diener
Derashot HaRan
But there is a question here, for after the splitting of the Red Sea we find it written (Ibid 14:31): "And they believed in the L-rd and in Moses, His servant," the implication being that before this their belief was not perfect.