"turn on a dime" : to have a small turning radius; to change direction very quickly.

Being able to turn on a dime is a great thing for a sports car but a scary thing when it comes to the human heart. Many times in life, in our faith, in our relationships we can tend to turn on a dime.

Acts 14:18-19. In one verse a whole city's hearts turned on a dime. They crowd went from having to be restrained from worshiping and offering sacrifices in honor of Paul and Barnabas to actually stoning Paul.
Paul went from hero to zero. They turned on a dime.

Acts 28:4-6. The apostle Paul survives a shipwreck, only to be washed up on the shore of Malta. Once on land he is bit by a deadly viper and the natives think he must be a murderer receiving justice for his ills. Moments later when they see that he hasn't dropped dead from the venomous bite, they turn on a dime and deduct that this man "must be a god."

Now, Paul goes from zero to hero. They turned on a dime.

The Son of God was no stranger to this kind of treatment.

Jesus was welcomed into Jerusalem on a Sunday. The crowds are shouting His praise only to turn on a dime and begin yelling "Crucify Him!" "Crucify Him!" five days later.

Jesus went from triumphal entry to torture and execution. They turned on a dime.

God knows our hearts can be extremely fickle. Any measure of faithfulness in our lives is a direct by-product of the grace of God.

It's not a bad idea to pray something like this on a fairly regular basis, "Lord hold my heart steadfast because we both know I'm so prone to wonder and turn on a dime at any moment."