"Even so faith, if it hath not works, is dead, being alone." (James 2:17)
You gotta know that the un-Christian Christian is an enigma to the unbeliever. Here you have a human being (just like them) who subscribes to the worldview of, arguably, one of the most moral teachers in human history (some people have a hard time with Luke 14:26)—certainly the most selfless. They take their rules and rubrics from an ancient tome called "The Holy Bible", something that they had no hand in producing. And they join together with a bunch of other like- (close-) minded individuals at a "church". And yet, they don't live it out in their behavior. They don't put works to their faith. Two strikes and they're out.
Many people who don't believe are smart enough to work through the finer points of theology from a mental standpoint. They can wrap their minds around a spiritual realm. They can visualize the concept that "God" would have been the one to set up the clear black-and-white of "good and evil". But here's the thing, just knowing it in your head isn't enough. Same goes for Christians who believe that Jesus died and rose from the dead. From head to heart is essential.
For some, you lost them at "God". That's the first strike. But when you have a Christian who is born again and yet not walking in their faith with honest heart and pure motive, that's the deal breaker. When Jesus says "By this shall all men know that ye are my disciples, if ye have love one to another." (John 13:35) and we don't fulfill that tautology (in Logic, either this or that) as laid out by Christ, then all bets are off. The time for playing church and playing around is over. Ever since the Enlightenment (and before, in pockets), man has sought to replace a traditional biblical approach to the world within and without with "perfectly rational explanations". And, it would seem, the one thing Christians could do to allay and alleviate the encroaching tide of unbelief, is the one thing at which they've failed miserably. Namely, love. I.e. know Jesus.
Love is the most overused and least understood concept in the world today. It begins (for us) with Jesus: "As I have loved you..." (13:34) This is the grand failure of modern-day Christianity. To fail to see the love of Jesus and to in turn live it out to the rest of the world. A simple statement with a thousand attendant variables. All of which are answered and silenced by His love. He lives to love. Think about it. Then live it out.
"Ye are the salt of the earth: but if the salt have lost his savour, wherewith shall it be salted? is is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men." (Matthew 5:13)
The idea here is that Christians (the 'salt' of this parable) are good for one thing only. To be "witnesses unto [Jesus]" (Acts 1:8). Failing that, we fail the most rudimentary and fundamental particle of our life. We are meant to reflect the character of Christ in all we do and with everyone we interact with. They're watching. In the backs of their minds, they may think us to be fools. At our very cores, we are sadly deluded (and dilluted) and therefore can only go so far in the realm of human existence and experience. Know this. And love them anyway. Show them by your actions and thoughts that Jesus is real to you. They'll know. That thought that they hide that says that we're crazy will acquire a strange bedfellow. They might think we're crazy and blind, but they'll also see Jesus all about you. Over time, the conviction of the Holy Spirit will grow and consume them, softening their heart. And like a natural gas leak, the Holy Spirit will enshroud them and upon one spark of belief will ignite and rebirth their spirit. All because you loved them with the love of Jesus.
No pressure. It's up to us. Fortunately, we have all the power of the Holy Spirit to help us and testify to the truth of our actions.