Moses said unto the people, Fear ye not, stand still, and see the salvation of the LORD...The LORD shall fight for you, and ye shall hold your peace. Exodus 14:13, 14.
Here you find the Israelites were commanded to stand still and see how God would deliver them from the Egyptians. However in this case the Israelites did not have any choice. The only way available for them was to cross the Red Sea. If Pharaoh's army was not chasing them, they would have never crossed the Red Sea. To learn more about this, please read Are you a partner in God's promises.
However we find a similar promise once again in the Bible.
Ye shall not need to fight in this battle: set yourselves, stand ye still, and see the salvation of the LORD with you, O Judah and Jerusalem: fear not, nor be dismayed; to morrow go out against them: for the LORD will be with you. 2 Chronicles 20:17
This time it was King Jehoshaphat and Israel and this time it was obeyed differently. King Jehoshaphat and his people obeyed the command in the correct way. King Jehoshaphat urged the people to trust God and His prophets and instead of going to war in a conventional manner he appointed singers unto the LORD, and that should praise the beauty of holiness, as they went out before the army, and to say, Praise the LORD; for his mercy endureth for ever.
King Jehoshaphat has showed us how to obey God's commandment and respect His promises. When God says something He will do it. God will fulfill what He says whether we believe Him or not. But when we believe Him we become instruments of His glory and we experience God's power in a new way and it transforms us and makes us powerful Christians.