09/25/2013 at 05:00 PM EDT
As leader of Saddleback Church – a multisite Christian mega-church based in Lake Forest, Calif. – and author of the worldwide best-selling spiritual guide The Purpose Driven Life
, Pastor Rick Warren has ministered to millions.
But after his 27-year-old son Matthew took his own life
in April, with a gun purchased on the Internet, Warren and his wife Kay began a grieving process that forced them to wrestle with tough questions of faith.
"I asked, 'Why God? Why did you allow this?' " says Warren. " 'Why have I prayed every day for my son to be healed and it didn't happen?' I've seen miracles. But they are called miracles because they don't always happen.''
Now, after four months away from his ministry, Warren, 59, has returned to the pulpit, sharing with his congregation Matthew's lifelong struggle with depression and mental illness. In a PEOPLE exclusive, Rick and Kay, spoke of their pain and hope.
Matthew, the youngest of three children, was diagnosed with depression at the age of 7, but years of therapy and medication proved fruitless.
"He was seeing psychiatrists early on, but he was misdiagnosed," says Kay, 59. "They said he was bipolar for years. It wasn't until a year and a half before he passed away that we came up with Borderline Personality Disorder. By that time he was jaded."
For more on Rick and Kay Warren's emotional and spiritual journey, pick up this week's issue of PEOPLE, on newsstands Friday