John Wesley on Exodus 14:19

The angel of God - Whose ministry was made use of in the pillar of cloud and fire, went from before the camp of Israel, where they did not now need a guide; there was no danger of missing their way through the sea, and came behind them, where now they needed a guard, the Egyptians being just ready to seize the hindmost of them. There it was of use to the Israelites, not only to protect them, but to light them through the sea; and at the same time it confounded the Egyptians, so that they lost sight of their prey, just when they were ready to lay hands on it. The word and providence of God have a black and dark side towards sin and sinners, but a bright and pleasant side towards those that are Israelites indeed.