2 Corinthians 6:14

Do not be unequally bound together with unbelievers (do not make mismatched alliances with them, inconsistent with your faith).  For what partnership can righteousness have with lawlessness? Or what fellowship can light have with darkness.  

Bad influences in relationships, books, media can lead the believer easily out of faith.  Doubt begins like a seed, just like faith.  Doubt is negative faith and starts by deviating from God’s word and believing a lie.  An unbeliever doesn’t have faith nor understands faith in God.  It doesn’t mean we aren’t to associate with unbelievers but not to make alliances that lead us out of faith.