Israel saw G-d's mighty arm…they believed in G-d:
Generally, after seeing something, we no longer need to
accept it on faith. If so, after seeing G-d's great hand, why
did the Jewish people still need to believe in G-d and Moses?
The answer is that once they had seen and validated what
they had previously only believed in, they were able to "upgrade"
their belief and believe in that which remained beyond what
they had seen; they believed in what they had not seen.
The potential to do this is infinite. We struggle to perceive what
we currently believe; when we succeed in doing so, we realize
that there is still more about G-d to believe in. In this way we
perpetually broaden the horizons of our faith and our capacity
to relate to G-d on ever-higher levels.
—From the Kehot Chumash