I think one way to look at this question would be to ask ourselves how often we would quote Scripture in our everyday lives. Very rarely, I'd imagine. We want our lives to speak for themselves. An unbeliever will be much more likely to ask us about our faith when we're kind when others are mean, go out of our way to love the unlovely, and keep a smile on our faces, than if we pound a Bible over their heads.
I think this is true in novels as well. Let your character's lives speak for themselves. If the opportunity comes up to share a Scripture (just like in real life) by all means share it, but make sure you're not quoting Scripture just to look smart. And I say all this recognizing the great power in the Word. After all, the gospel [good news] is the power of God for the salvation of those who believe.
"A man finds joy in giving an apt reply, and how good is a timely word!" Proverbs 15:23