How do we lose our way?

An old friend recently shared with me that he has lost his way. He wants to believe but can't. He believes in God but can't trust a Bible that was 'written' by men. So many of my friends seemed to go this route and I often wonder what caused it. Were they ever truly saved by grace? Or were they following a 'religion' instead of Jesus.
I told him I walk with Jesus...that's my goal. To serve Him with my life. It's hard sometimes, especially when you feel alone in this world. But I know God is good. His Word tells us that 'they stumble because they disobey the word, as they were destined to do' (1 Peter 2:8b) But we can rejoice because we are 'a royal priesthood, and a chosen race....that you may proclaim the excellencies of him who called you out of darkness into his marvelous light.' It's hard for me to understand how someone falls away. Why would you want to go back into the dark? ...that horrible, dreary, sad and lonely place.....when you have seen His marvelous light? His love, His compassion, His forgiveness. It pains my heart to see this happen. And all I can do is be faithful in my witnessing and pray for these people, and love them.
I carry this burden for a few family members too...and I believe I must remain steadfast until the day of Christ to love them and try, by God's grace and help, to pull them back into the marvelous light.