see the light
see the light {v. phr.}, {informal}
To understand or agree, often suddenly; accept another's explanation or decision.
I did not approve of his action, but he explained his reason and then I saw the light.
Bill wanted Harry to help him, but Harry wasn't in the mood until Bill offered to pay him. Then Harry saw the light.
Mary thought it was fun to date older boys but when they started drinking, she saw the light.
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Source: A Dictionary of American Idioms