To forfeit salvation is to willingly give it up, so to speak. One decides to stop following Christ. This is not something that happens accidentally. One cannot "lose" salvation as in how someone might lose his car keys.
Now, as has been pointed out, there are some different schools of thought about this. One view is that once someone makes a conscious decision to stop following Christ/turn away from the faith, he is forever in a lost state with no second chance of being saved. Also, a person like this is then believed to have no desire to be saved.
Another view is one where a person may fall back into a life of sin. This is not the same as failing occassionaly and committing sins sometimes. This would be someone that basically keeps on committing sins without asking repentance. This view would not necessarily say that the person sinning is what caused him to no longer be saved. But rather that the sin would be a symptom of the "disease" - of that the person had fallen away from Christ. This view believes that the Bible teaches that one can come back to Christ in this situation.
Then again, some that suscribe to the second view also believe that there is a point one can reach where a person apostatizes to where he is basically in the condition of the first view of being forever lost.
Then there are those that believe that once saved, if one commits a sin (and sometimes these are only certain kinds of sins), he is lost until he repents of that sin.
Now, as has been pointed out, there are some different schools of thought about this. One view is that once someone makes a conscious decision to stop following Christ/turn away from the faith, he is forever in a lost state with no second chance of being saved. Also, a person like this is then believed to have no desire to be saved.
Another view is one where a person may fall back into a life of sin. This is not the same as failing occassionaly and committing sins sometimes. This would be someone that basically keeps on committing sins without asking repentance. This view would not necessarily say that the person sinning is what caused him to no longer be saved. But rather that the sin would be a symptom of the "disease" - of that the person had fallen away from Christ. This view believes that the Bible teaches that one can come back to Christ in this situation.
Then again, some that suscribe to the second view also believe that there is a point one can reach where a person apostatizes to where he is basically in the condition of the first view of being forever lost.
Then there are those that believe that once saved, if one commits a sin (and sometimes these are only certain kinds of sins), he is lost until he repents of that sin.