Perhaps someone is a truly decent person, who has helped us grow spiritually. So we might return the favor with our generosity. Perhaps we even think that being generous enables that person to help others. And perhaps that person has flaws, but we think that there is more good than bad, so we overlook the bad. Then again, sometimes the person does more bad than good, yet in wanting to see only the good, we might think that eventually the person will get back to doing good work, and we keep supporting that person. Then perhaps it's been a long time since that person has done anything worthwhile, and we keep on giving, thinking that our goodness will set an example, and get the person back on track. And where exactly in all of that did we cross the line, from enabling a good person to do more good work, to feeding the Devil?
First and foremost, we all must have a personal relationship with God, and we must learn to get our information directly from the source, which is none other than God. If we are open to the truth, and if we ask God if helping someone else is the right thing to do, we will get the truth, and we'll know what to do.
Second, we must never forget to use common sense. If an evangelist needs a new car to take the ministry to a broader audience, we might take a second look at the car that he/she already has. If it's nicer than the cars that we drive, there might be something fundamentally wrong with the ministry.
What, in fact, is a ministry? It's people talking to people. Why would we pay someone else to do this for us? If the beliefs are not powerful enough for us to understand them, and use them in our daily lives, and if we cannot explain them to others, the ideas probably are not worth propagating anyway. So we do not need to build new churches and schools, nor do we need to go on any road trips to take our message to a broader audience. We need to listen, think, and talk. There are plenty of churches and schools already in existence. We'll just use those, if our message is spreading through the general population, as it will if it is good. But we certainly don't want our "message" spread by selfish, manipulative people who will convince the wise that this system is a scam.