Affeldt is a devout Christian and a strong advocate for ending child poverty. He writes a weekly blog about his Christian faith and his desire to stir a movement in the cause of helping the suffering and marginalized. In October 2010, during a visit to a Bay Area elementary school alongside former Giants pitcher Juan Marichal, Affeldt told the students, "I try to make sure everybody, all the young people in the world, also have a chance to dream really big." In 2009, Affeldt donated five thousand dollars to the Not For Sale campaign for opening a medical clinic in Thailand for former child slaves. He also became a prominent member of Not For Sale's Free2Play campaign, pledging a $100 donation for every strikeout and recruiting other professional athletes, including his teammate Matt Cain and the St. Louis Cardinals' Matt Holliday. In addition, Affeldt has lent his support as a spokesperson for Larkin Street Youth Services, a San Francisco non-profit that provides services to young people experiencing homelessness.