Israel, who displayed such unbelief before crossing the sea, believed in God and Moses after they were delivered from Egypt. I think this is true for nearly everyone, even people who are religious prior to actually being delivered from their sin. Human nature is such that we don’t believe until after we are delivered. I think God designed us this way on purpose, so that He receives glory and we cannot boast.

I love God’s statement to Moses in verses 15 and 16. He asks Moses why he is crying out in prayer and commands him to tell the Israelites to go forward. He is to lift up the staff and divide the sea to walk across on dry ground. This is one instance in Scripture God instructs someone to stop praying. Prayer is good. We ought to maintain an attitude of prayer without ceasing (cf. Ephesians 6:18; 1 Thessalonians 5:17). Sometimes prayer can keep us from moving forward in the faith and in action. Some people use prayer as an excuse for inaction and think it spiritual or godly. It is not. Sometimes we need to stop crying out to God and move forward like God instructs Moses here. 

God again says He will harden Egyptian hearts in order to receive glory for Himself. He does. God defeats the Egyptian army and delivers Israel.