At issue here are a couple of things. First, when we read the NT description of salvation, we have to ask whether an author is addressing the front end or the back end. James addresses the back end; Paul usually addresses the front end. When an author addresses the backend, he is asking questions like Are you behaving as a Christian should? The frontend question is asking, What must I do to be saved?
Now, there is another layer to this whole thing that needs to be addressed. The scriptures seem to speak very clearly about eternal security: that is, that a believer can have the assurance that he is saved right from day one. It is difficult to harmonize this teaching with anything that suggests we can lose our salvation, or any teaching that suggests that we need to do works to stay saved. Eph 2.8-9, Rom 8.35-39, John 10.28ff., are among the key texts here.
This leads to the question of what Christ accomplished on the cross. The fundamental question we have to ask is this: Is the death of Jesus sufficient to pay for all of my sins--past, present, and future, or do I need to add to his work? Put differently, Is there any work I can do to get myself saved or to keep myself saved? Frankly, anyone who believes that there is something that we can add to the cross of Christ is blaspheming. It's that important to get this one right.
Now, does this mean that a person who prays a prayer can live like the devil? Hardly. The scriptures are also clear that saving faith involves repentance. This means that a person looks back and changes his mind and his heart about his sins. Without repentance, there is no saving faith.
There is another layer to this discussion: it is the role of the Holy Spirit. Rom 8.16-17 affirms that the Holy Spirit convinces us that we are saved. This witness of the Spirit is something that takes place right from the first. But with his witness comes his work: he begins to work on our hearts, causing us to desire to live for God. So, from one perspective, it is possible to say that without works one's salvation is in question. Those works, however, do not save a person, nor do they keep him or her saved. Rather, they are evidence that the Spirit is at work and therefore that the person truly believed.
So, how does all this look pragmatically? It means that perhaps you could question whether your brothers really put their faith in Christ in the first place. They cannot simply get fire insurance and forget about their Lord. If they truly believed, the Holy Spirit will not let them do this! They will be miserable. But it is crucial that you not add to the work of Christ as you wrestle with whether your brothers are truly saved. Doing this could put you in a worse position than they!