Discovering bitter water at Marah was a devastating experience for the Israelites after their deliverance from Egypt. They were totally unprepared for it.
We may have bitter experiences too - disillusionment, business failure, broken relationships, a health breakdown etc. God will often use these to test and strengthen us.
Derek explains God's purposes in testing and how to respond. Complaining will produce nothing; persevering in faith will bring deeper revelation of Himself. By responding in the right way we will discover that man's disappointments are often God's appointments.