John consistently gave his reasons for writing. The purpose for his Gospel was "that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name" (20.31). The purpose for The Revelation was "to shew unto his servants things which must shortly come to pass" (1.1). The purpose for this Epistle was "that ye may know that ye have eternal life" (5.13). Those who "believe on the name of the Son of God" possess this confident knowledge. As stated, John's Gospel is geared to promoting the faith that brings eternal life; the Epistle is written to give believers the assurance that they have it.
John never suggests that ecstatic feelings provide that assurance, but rather what is "written". The blood of Christ provides salvation, but the Word of God imparts assurance. Doubters have often been asked, "Whom are you doubting?". God's promises are reliable; "the scripture cannot be broken" (Jn 10.35). To take God at His word is rational; to accept what He undertakes sweeps away the clouds of doubt.
John has no issue about people claiming to have eternal life; earlier, he wrote to the whole family of God, "your sins are forgiven you for his name's sake" (2.12). Sometimes religious unbelievers charge God's people with presumption for claiming to be saved. The claim would be audacious if salvation depended on contributions from us. The fact is that forgiveness is "for his name's sake", meaning that the believing sinner can ally himself with Paul and speak about "God; Who hath saved us" (2 Tim 1.8-9). To doubt it is to cast aspersions on the veracity of His Word.
Confident that our relationship with Him is right, we have additional "boldness…toward him" (5.14, RV) in the matter of prayer; "boldness" again carries the concept of freedom of speech. The verse presupposes sensitivity in prayer, asking "according to his will". It is possible to "ask amiss" (James 4.3). Selfish and unspiritual requests will fall on unsympathetic ears. Generally, the will of God is discerned from the Word of God, although there are matters of personal circumstance for which there is no Biblical directive. We need guidance for the huge issues of life, marriage, employment, location etc. In seeking God's will it is vital to keep the following principle in mind: "Trust in the Lord with all thine heart; and lean not unto thine own understanding. In all thy ways acknowledge him, and he shall direct thy paths" (Prov 3.5-6). Willingness to acknowledge Him by making His interests paramount allows His will to be worked out, and so we pray, "Thy will be done in earth, as it is in heaven" (Mt 6.10).
One aspect of prayer is intercession, praying in the interests of others. Ham saw his father sinning, and he told his brothers (Gen 9.22). Here, John anticipates us seeing a brother sinning, and we tell his Father! An exception is anticipated when no prayer should be offered, because the sin is "unto death". Evidently, John is thinking about the antichrists again, the apostates, and for them there is no way back. The Epistle to the Hebrews makes clear that there are people who give intellectual assent to truth about Christ and pass that off as saving faith. As time passes, they reverse their opinion, repudiate their former position and militantly promote erroneous notions. Such were the antichrists of John's acquaintance. Of people like that Scripture declares, "it is impossible…to renew them again unto repentance" (Heb 6.4-6); it is "sin unto death".
Some may argue that the one who commits the "sin unto death" is a "brother", and if so, a genuine believer and not an apostate. However, it should be remembered that throughout the epistle, the term "brother" is used to describe those claiming to be in the family. For example, John says earlier, "Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him" (3.15). Externally there appears to be a brotherly relationship between the two parties but obviously that is not the reality since one of them does not possess eternal life. Thus, it is consistent with the tenor of the epistle to suggest that the "sin unto death" is committed by an apostate and not a true "brother". Prayer would not be "according to his will" if we requested the restoration of such.
In general though, on observing a brother sinning our duty is to pray for him. Samuel said this to wayward people who were trying to sideline him, "God forbid that I should sin against the Lord in ceasing to pray for you" (1 Sam 12.23). It is worthwhile praying, for while every aspect of injustice, dishonesty, untruthfulness, immorality and evil speaking is sin, it is not "unto death" (5.17). Failure need not be final; recovery can be effected; restoration can be experienced.
We know (5.18-21)
In an epistle speaking much about knowledge, we now have another little cluster of things that "we know" (5.18,19,20). First, John restates truth that he had explained formerly (3.9); those who are "born of God" are not in the habit of committing sin. He had just been indicating that we could observe a brother sinning, but again he stresses that it is not normal for a believer to sin. Born again people are not addicted to sin. True, they are under pressure from "the evil one" (5.18, RV) but they can remain impervious to his attentions: "he that is begotten of God keepeth himself". For the sake of consistency, "he that is begotten of God" must still mean the believer, but is it conceivable that the believer "keepeth himself"? In the ultimate we are "kept by the power of God" (1 Pet 1.5), but God has made provision for our spiritual preservation. For example, there is "the whole armour of God" (Eph 6.13), and using such available resources contributes to us keeping ourselves. Again, "his seed remaineth in him" (3.9); we have "the divine nature" (2 Pet 1.4); we are "led by the Spirit of God" (Rom 8.14). So while it is unimaginable that left to his own devices a believer could "keep himself", every facility has been made available to us to that end. That being the case, the evil one cannot get a real hold on the child of God, as is the concept of the word "toucheth".
The second area of knowledge is the fact that there are two strands of humanity (5.19), "we" believers, and "the whole world", people who are still unregenerate. Believers are "of God", begotten of Him and taking character from Him. "The whole world lieth in the evil one" (RV). He is "the prince of this world" (Jn 12.31), "the god of this age" (2 Cor 4.4, NKJV). Who dictates the world's fashions, orchestrates its trends, promotes its attitudes, and encourages the idolising of its stars of stage and sport? The evil one. The whole culture of the world is permeated by wickedness and he is the one who manipulates the system, but, as v.18 has indicated, the believer can be outside his sphere of influence and immune to his attentions.
Another thing that "we know" is "that the Son of God is come" (5.20). Previously, John indicated that He had been "manifested", implying a former existence. The purpose for that manifestation was "to take away our sins" (3.5). He also tells us that He had been "sent", implying obedience to His Father's directive. The purpose for that was to be "the Saviour of the world" (4.14). Now the verb that is used is, "is come", implying a voluntary act. The purpose for that was to "(give) us an understanding". His coming has brought enlightenment, leading John's readers to a saving knowledge of God, the true God. This is in contrast to their former devotion to deities like Diana of the Ephesians surrounding whom there was only myth, pretence and superstition.
The saving knowledge of the true God brought them into living union with that God and with His Son, of whom John makes another unequivocal statement: "He is the true God and eternal life" (5.20, JND, ESV, Newberry etc). "His Son Jesus Christ" is the immediate antecedent of the pronoun "He", so Christ is identified as "the true God"; it is a statement of His deity! Again, linked with Him is "eternal life". He is the personification of that life (1.2) and its source and channel, for "He that hath the Son hath life" (5.12).
The penultimate verse would have been a fitting climax, but, having magnified Christ, John ends with an apt corollary, "keep yourselves from idols". Whether he is referring to the intellectual idolatry of the Gnostics, or a more general tendency to displace Christ from prime place in our lives, the message is clear; the magnetism of "The God of glory" (Acts 7.2) should attract us from all that is false and transient and secondary.