[Michael] Phelps told ESPN that [Rick Warren’s] book "turned me into believing there is a power greater than myself and there is a purpose for me on this planet."
Warren's book also convinced Phelps to reconcile with his estranged father, Fred, who divorced his wife when Phelps was just 9 years old. When the two men saw each other for the first time after so many years of separation, they embraced in a big hug.
After he left Meadows in November 2014, Phelps resumed training for the Rio Olympics.
Source: Hazel Torres - As quoted in Christian Today, http://www.christiantoday.com/article/legendary.u.s.swimmer.michael.phelps.reveals.how.purpose.driven.life.by.rick.warren.saved.his.life/