The following is from Authentic Christianity, by Martin Lloyd-Jones, regarding Acts 2:37–47.
Let me put it quite simply like this: What would your answer be if I gave you a sheet of paper and a pencil and told you to put down in as few words as possible your idea of what it means to be a Christian? […]
There are some people who quite clearly think that Christianity operates solely in the realm of the intellect. These are serious and able men and women who are concerned about life and its problems. They know that here is a traditional teaching, and they believe it their bounden duty to consider it. So they read about the Christian faith and may become very interested in it, even accepting a good deal of it. But it is all in the mind. It is all theoretical. They may greatly enjoy their study of Christianity; it may become their hobby, but it is nothing beyond that. In addition, many people devote their lives to theological study. These scholars and academics spend their time in intellectual argument, taking up religious issues and writing their books one against another or in agreement with one another. That is their whole life. […]
At the opposite extreme, there are those for whom Christianity is purely a matter for the feelings. They have had a wonderful experience of peace or love or happiness, and they say they need nothing else. The intellectuals, of course, condemn such people. “It’s pure emotionalism,” they say. “They cannot argue seriously with you. They haven’t read the books and cannot discuss them with you. They live on the wonderful feeling they say they’ve had and deliberately try to work it up again and again.” And, of course, there is a good deal of evidence that lends considerable weight to these objections.
Then there is a third group that puts the entire emphasis upon the will. According to this view, what makes a Christian is not what people think; and if they like to play with the emotions, let them do so. Rather, they say, whether or not you are a Christian hinges upon what you do. It is the way in which you live that is the deciding factor. Are you living for the good of humanity? Are you ready to make sacrifices? Are you ready to put desire for a great career on one side in order to do something heroic and wonderful and sacrificial? That is what makes people Christians. It is a question of making a deliberate decision to improve the lot of humanity and uplift the human race. This may take you into politics or into social work—the sphere is unimportant. As long as you are giving yourself in service, what does it matter what you believe? The intellect is comparatively unimportant. Indeed, you can be certain of very few things in a world like this. The important thing is your will and your desire and what you are actually doing.
A fourth view of Christianity, a view commonly held by many people who have been brought up as Christians — I myself held it for many a year — is the view that being a Christian is a task that you have to take up and that you take up more or less reluctantly and miserably in a spirit of fear. Christianity is mainly something that spoils life. You know other people who were not brought up as Christians, and you see that they do things freely without any hesitation at all, and you wish you could be doing the same things, but you are afraid. You have been brought up in a chapel or a church, brought up as a Christian, as it were, and though you want to do these things, you cannot. This Christianity stands between you and them. […]
So we are considering these eleven verses from Acts, and we see what an utter travesty this last view is of Christianity. This is what Luke wrote: “They, continuing daily with one accord in the temple, and breaking bread from house to house, did eat their meat with gladness and singleness of heart, praising God, and having favor with all the people” (vv. 46–47). Could anything be a greater contrast? This is Christianity.[pg. 64-66]