I'm not really a religious person, but I can see why people with substance abuse problems gravitate towards a belief system.
In most branches of Christian theology, you are redeemed of your past sins the moment you turn your life over to God. This is because Jesus's death on the cross is payment for your friends' poor decisions. A lot of people honestly have a hard time living with themselves once see how messed up their past lives were and because of that it makes an ideology such as Christianity more appealing to a recovering drug addict.
A lot of drug and alcohol wellness programs do put the idea of faith out there. Sometimes if it's an inpatient program you'll have bibles or the counselor will encourage people to balance the 7 areas of wellness (which includes spirtuality). And often you'll have a "saved" former addict in a substance abuse support group. However, it is something that does work sometimes and I have seen friends who have kicked the habit because of Jesus. But both of them ended up turning into super judgmental and hateful jerks