"Our hearts are battered, but they will mend. The world is broken, but it will be healed in the end. And good will prevail as the One God of many names will prevail," his mother, Paula Kassig, said at a news conference at Epworth United Methodist Church in Indianapolis.
Kassig's father, Ed, added, "Please pray for Abdul-Rahman, or Pete if that is how you knew him, at sunset this evening. Pray also for all people held against their will in Syria, Iraq, and around the world. Lastly, please allow our family the time and privacy to mourn, cry -- and yes, forgive -- and begin to heal."
Kassig, the fifth Western hostage ISIS has claimed to have killed since August, changed his name to Abdul-Rahman after converting to Islam, according to his parents. The aid worker and former Army Ranger was abducted in October 2013 while traveling to a town in eastern Syria.
Kassig's parents had issued a statement on Sunday night asking that their son be remembered for what he did with his life, not how it was ended.
"We are heartbroken to learn that our son, Abdul-Rahman Peter Kassig, has lost his life as a result of his love for the Syrian people and his desire to ease their suffering," they wrote. "Our heart also goes out to the families of the Syrians who lost their lives, along with our son."
Towards the end of the nearly 16-minute video, a militant stands over a severed head, saying, "This is Peter Edward Kassig, a U.S. citizen, of your country; Peter who fought against the Muslims in Iraq, while serving as a soldier."
The militant speaks with a British accent and the video identifies his location as Dabiq, a small town in the northern Syrian province of Aleppo, near the Turkish border. This is the first video in which the hostage isn't seen alive before his apparent murder.
Paula Kassig also said her son's "life is evidence that he’s been right all along; one person can make a difference."
The family has asked that contributions in Kassig's memory be made to the Syrian American Medical Society, which is working to meet the medical needs of displaced Syrians.