Parents, if your children go astray spiritually, do not beat up on yourself. Reproach not thyself. If perfection is the litmus test for parenting, we all fail. We can only do our best, plus pray mightily and frequently.
Outside influences beyond our control are pressuring our children. Pornography is omnipresent, easily available. Drugs are proliferating. Mental illness is increasing. One wrong person coming into the trajectory of a teen's life at the wrong moment can undo everything. By the way, on the other hand, the right person can also affect radical change for the good. I urge you to seek ways to enlist Godly adults to disciple your older teens.
Freewill comes into play. However good a job we do as parents, we must remember; once children become adults, they are responsible for their own behavior and decisions. Young adults have to make their own choices.
We have to face the painful fact some were never believers. Children sometimes go through the motions of becoming a believer, but we don't have a lens to look into a child's heart to see for sure what happened there.
The child/adult transition age continues to get younger. It was 18; then it was 16, drivers license age; now it comes with the first cell phone. Studies are showing that sexting has become a problem for children as young as 12.
One helpful adage is; rules without relationships can cause rebellion. In this equation, once our adult children are no longer under our rules, and already in rebellion, only one thing is left: relationships. Parents, do all in your power to be your teens and adult children's best friends. Once they know where you stand on a given issue, try to bond. Don't compromise, but don't hold at arm's length either. When you disagree, talk but do not badger.
We are not insiders able see everything. As we see situations, we can react like Pharisees, with a condemning frown, or be like Jesus, who leaned toward compassion. Henceforth, I want to more and more be like the latter.