The definition of faith is complete trust in something. The reason people lack faith, or lose faith, is because they either have less-than-complete trust, or no trust.
In the case of those who lack faith, they have certain things they believe, but they’re never willing to try out those beliefs on the real world. To them God and faith and Christianity is something that’s in your head; or in other words imaginary. “Spiritual,” to them, means “not real.”
Confront them with this and they’ll protest: “No, I do so believe that Christianity is real.” As to why they don’t act like it, they’ll make some excuse about how they don’t have “the gifting” or “the anointing” or “the leading” to do such a thing—something in Christianese that sounds like they’re waiting for His approval or say-so, but actually means that they won’t do jack squat until Jesus or some angel appears to them personally and orders them to act like a Christian. They’re waiting for that. Until it happens (and they’re fairly sure Jesus doesn’t do such things anymore anyway) they’ll assume the instructions in the scriptures, those that are meant for every Christian, don’t necessarily apply to them. And there are plenty of Christians much better qualified to minister; besides, God does all the heavy lifting anyway.
As you can see, the fruit of doubt is apathy. (What about activist atheists? Oh, they definitely have faith—they completely trust that God doesn’t exist—which is why they act. Notice how agnostics, who have no such faith, are rarely so outspoken.) When we don’t really believe in God and Christianity, we don’t act like theists and Christians. We’re spurred to act by what we do believe in. Even if we’re trying hard to avoid it, we’re at least in action.
This all bounces back to the definition of faith in Hebrews:
Faith is the confidence that what we hope for will actually happen; it gives us assurance about things we cannot see.
—Hebrews 11.1 NLT
If we’re confident that we’ll get what we expect, we’ll do what it takes to get that. If we’re not so confident, we’ll either do a half-assed job, or we’ll not even try.
So, if you’re not acting, it means you lack faith. If your fellow Christians are not acting, it means they lack faith. If your church has no ministries, no forms of outreach, nobody’s contributing or donating towards its upkeep, and poorly-attended prayer meetings, it means your church lacks faith. “Faith by itself isn’t enough. Unless it produces good deeds, it is dead and useless,” James put it, (Jm 2.17) and well put.
How do we turn this around, then? Start doing.
It’s a very common misconception among Christians that we need to have faith before we act. If you think about it, this doesn’t make sense. That’s not how we do things in real life. We try things. The first time I took judo, I had no idea whether I could successfully throw someone. But I gave it a try. Took a few attempts. The same is true with Christianity: We may not get it right the very first time, but practice makes us better at it. Yet somehow people have this belief that we can’t act until we believe; that it’s hypocrisy if we try to act without believing. Hogwash. It’s only hypocrisy to say you believe when you don’t. It’s not hypocrisy to say, “I don’t know if this’ll work, but I’ll give it a shot.” It’s not hypocrisy to tell God, “I do believe, but help me overcome my unbelief!” (Mk 9.24) He will: When we act, He acts.
Christians regularly claim that we’ll get instant results once we start acting. They’ll promise a renewed purpose, much stronger faith, supernatural experiences, visitations from God, better finances, and all sorts of things. They shouldn’t. The scriptures promise no such thing; the only thing Jesus warns us to expect for our obedience is persecution. I make no guarantees about what acting upon your faith will do for you, and any Christian who does so is irresponsible. The only thing I will say is that when we’re walking God’s path, it’s a lot easier to hear His footsteps. Your faith can grow as a result of good works. How much it grows is kinda up to you.