NEW YORK -- During his eulogy for the Rev. Clementa Pickney, one of nine worshipers killed at Emanuel AME Church in Charleston, S.C., a visibly moved President Barack Obama paused as he pondered mysteries of grief and forgiveness.

"Blinded by hatred," he said, the gunman could not comprehend the "power of God's grace. … Amazing grace. Amazing grace." The president then began singing: "Amazing grace! How sweet the sound, that saved a wretch like me! I once was lost, but now am found, was blind, but now I see."

The congregation joined in during that June 26 service, which was not a surprise since researchers say believers worldwide sing the Rev. John Newton's classic at least 10 million times a year.

But the president's solo had an unexpected impact in New York, where the cast of the Broadway musical "Amazing Grace" was doing preview performances before its July 16 opening in a tough town for a show about sin, repentance and salvation.

Some theater insiders, for example, had suggested changing the show's name.

"When Obama sang the song it was like heaven for us. If the president knew this song, that meant it was acceptable, that it wasn't just something for church people," said veteran playwright Arthur Giron, who wrote the musical's book -- dialogue and many lyrics -- along with self-taught composer Christopher Smith.

"You see, many Broadway people didn't know 'Amazing Grace,' let alone what the song was about. They obviously didn't know the story of the song and that was the whole point of our show."