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Today's chapter begins with yet another restatement of the three tests of authentic conversion. God's true children believe that Jesus is the Christ; they love God's other children; and they obey the commandments of their Father (5:1-2). John reminds us that God's commandments are not burdensome due to the fact that we've "overcome the world" (5:4). That is, because we've been spiritually reborn, we've been delivered from our addiction to the world's sinful system and possess a God-given ability to obey.
What did John mean when he stated that Jesus "came by water and blood...not with the water only, but with the water and with the blood" (5:6)? Historical sources indicate that in John's day, there were those who taught that Jesus was born a mere man, but that Christ descended on Him at His baptism and then departed from Him just before He went to the cross. John refuted this heresy by affirming that Jesus was the divine Son of God when He submitted Himself to baptism (through water) and when He died on the cross (through blood). Jesus and Christ were not two separate persons; notice how John referred to Him as "Jesus Christ" directly in the middle of the verse under consideration. He was from birth to death and forevermore both the man Jesus and the Christ of God.
Years ago I used to quote 1 John 5:13 to every person who came forward to "receive Christ" in the church I pastored in order to "give them assurance of their salvation:"
These things I have written to you who believe in the name of the Son of God, so that you may know that you have eternal life.
I would ask each new convert, "Do you believe in Jesus, the Son of God?" When they replied affirmatively, I would say, "Then the Bible says you have eternal life." However, I began to wonder if I was making false affirmations when I noticed that so many new "converts" never returned to my church, indicating that they had no interest in learning how to follow the Jesus in whom they supposedly believed.
I realize now, of course, that John was saying in 5:13 that he had written his entire letter, which repeatedly listed the three tests of authentic conversion, so that his readers who passed those three tests would know that they have eternal life.
Passing the three tests not only gives us assurance of salvation, but also gives us assurance in prayer, as long as we ask for what is according to God's will (5:14). John cites two examples of prayer requests, one of which can be prayed with assurance and the other which can't. We can pray confidently for a brother whom we've seen commit a sin "not leading to death" and God "will for him give life" (5:16). But in regard to a brother committing a "sin leading to death" (5:16), we cannot pray confidently.
So what is "the sin leading to death"? Some (particularly those in the unconditional eternal security camp) think John was referring to a sin that would result in God's discipline in the form of physical death, but I don't know how we could discern if a certain sin was about to invite God's discipline in the form of physical death. And if we assumed that a sick Christian was being disciplined by God with sickness, how would we know if the ill person had committed a sin that ultimately would or would not lead to physical death?
It seems to me that the greater context indicates that it was spiritual death that John had in mind. So what is a sin that a believer can commit that leads to spiritual death? Remember that we read in the book of Hebrews that "it is impossible to renew to repentance" those believers who have "fallen away" after having reached a certain level of knowledge and spiritual maturity (Heb. 6:4-6). Certainly we would be able to discern if a fellow believer commits apostasy, and there is no sense praying for his impossible restoration. How I wish that this, and many other things in John's epistle, were more clear!