Once Saved Always Saved vs. James 5:19-20

James 5:19-20 reads "Brethren, if any of you do err from the truth, and one convert him; Let him know, that he which converteth the sinner from the error of his way, shall save a soul from death, and shall hide a multitude of sins."This passage is written to brethren, that is, Christians. It presents the possibility that one brother might err from the truth. If he does, other brethren are to try to convert him back. If they are successful, they save his soul from (spiritual) death. If they don't win him back, the implication is that his soul is not saved from death. So according to this passage, it is possible for a Christian to sin so as to lose his soul.That's pretty simple. God said it, so we should believe it.