The following is taken from the Book of New Testament Summaries

1. (A.D.85?) [Likely a circular letter written by John to all the Churches in Asia Minor where he ministered, with his home now in Ephesus since the fall of Jerusalem in A.D.70. The heresy of Gnosticism (denying that Christ came in the flesh while accepting the premise of His teachings) was beginning to entrench itself as a cult within the Christian community, and whose main proponent, John's arch enemy in the faith, also resided in Ephesus, now the Christian capital of the world. John writes to refute such damnable error, 2:21-26.] It is ludicrous to suggest such fallacy to John, a man of flesh who had been with Jesus of like flesh since being called at a relatively young age (likely late teens and youngest of the 12) from the fishing trade on Galilee to walk the dusty trails of Canaan, freeze together in its cold winter nights, bathe together in Jordan, dine together in the upper room, whose head had leaned upon a pulsating breast at that last table. NO! He would have none of this! These modern day "scholars," these "Johnny-come-lately" preachers were not there as he and others. They did not see what he saw; they did not touch as he had touched; they did not examine for truthfulness and subsequent belief and commitment as he had. There is no comparison between these false teachers' witness and testimony and his own. Theirs is hearsay at best, conjecture most likely, both inadmissible in any court of law, in any day; his was an eyewitness account, and that over three years, not to mention the confirming ministry and miracles of the Holy Spirit as Jesus promised subsequent to His departure, still evident in the lives of present day believers! NO! He will have none of this! (Who would you believe?) To John, this man Jesus spoke truth, pure Truth, and in Him he did not discover, nor detect, one shred of deception, fallacy, error, misrepresentation, inaccuracy: no, not so much as even a blunder, mistake, misquote, or misspeak! He is that True Light, God's Truth, come to shine in men's darkness, exposing error and sin while illuminating the way back to God for the sinner; and His Light is greater than men's darkness, incapable of being extinguished ( Jn.1:5,9). (See summary on James 1. John had reported in a recent similar writing [his gospel] that Jesus Himself said, "I AM THE LIGHT OF THE WORLD," and "I AM THE WAY, THE TRUTH, AND THE LIFE: NO MAN COMETH UNTO THE FATHER, BUT BY ME." Jn.8:12; 9:5; 12:46; 14:6. Ask the Apostle Paul of this Light, Acts 9:3-9; II Co.4:3-7; II Thes.2:3-12.) Knowing this personally to be true, and its wonderful truth the source of his own great joy, he is confident that when believed by others, it too will explode in their hearts, producing that same satisfying love and abundant joy. A sinner once blind, now made to see, once lost in darkness, now saved by the Father's own Son. Who among us could not help but be overcome with such relief, such wonder, such gratefulness, such joy at the absolute certainty of such Truth! And so he writes.
   
2. Knowing that the lust to sin resides in the flesh, and that the flesh is of the world, and as long as the flesh remains alive, as it has always sinned, it will continue to be predisposed toward. Anyone who denies this knows not the truth of God's Word. Moreover, since Christ came, He not only paid the price of our past sins, but all sin yet to be committed by the flesh as long as it remains alive! However, unlike before when we were alien to Him, His Spirit then not living within us, we now have His abiding presence, power, and forgiveness to overcome as He overcame, if we continue to abide in Him. Therefore, John writes to so encourage. Sin is the only condition that separates one from His fellowship, but not His love. Because His love continues, His forgiveness is possible when requested and that fellowship will be restored. So, when one finds himself fallen, as one on occasion will fall, believe His promise that He will never leave nor forsake, and immediately ask for His forgiveness which He will not deny. But be forewarned, as Christ was holy, He calls all to be holy. It is impossible to abide in Him who is holy, if His holiness is not continually clothed upon us (His forgiveness for our sins and His imputed righteousness). Do not believe and live as the Gnostics who think that the acts of the body do not have an impact upon the condition of the spirit, themselves living in revelry while maintaining their piety and superiority of spirit and knowledge. This was not Christ's life, nor should it be His true follower's. As He was anointed, so too are his true disciples. This they have known from the beginning; so, they are not to be persuaded by these latter day antichrists. It would be shameful to believe them now, denying Jesus' Earthly life, physical suffering, and victory, to discover your tragic mistake when He appears on the contrary. If He was righteous, then how can one claim to know Him and willingly remain unrighteous? This is not Christ's way! He not only makes us righteous through His atoning and anointing work, He gives to us the desire to be righteous as well. If one does not hate the thing that Christ hates—sin in the sinner or in one's self—then how can he be Christ's?
   
3. Having already introduced the dual nature in the argument of knowing if one is in Christ—hating that which He hates (2:15), and conversely, loving what He loves (2:10) — John continues on this theme, expanding now on the latter aspect. It is not enough to hate sin, but one must be obedient to His only commandment given ( Jn.13:34): to love the brethren, and not just the brethren, but one's enemy as well (for he too Christ died Mat.5:43-48), if one can be counted truly in Him. And whatever this one may appear to be at present, it is not known for certain, but will be revealed when Christ shall return; for both will appear in likeness, clearly of the same origin—the Father in Heaven. For this cause it is "natural" that he who is of Him now in this life, will do those things He Himself did while among us, manifesting His love for all men, every manner of sinner. As His works were righteous, the believer's will be as well; as He was compassionate, he will be likewise—God in him, manifesting Himself still to the world. This is our assurance that we are in Him, and have unlimited access for life's needs as He has given ( Jn.15:7).
   
4. John now gives clear testimony to the correct identity of the true believer and genuine teacher. There are many spirits in the world, the Holy Spirit having come at Pentecost being the authentic, but only He will manifest God's love toward all men, especially to the family of God. That one who denies that Christ came in the flesh for sinful men denies the price the Father paid in ordaining His only begotten Son to take away man's sin upon the cross through His physical suffering and death, denying the sacrificial price He paid through His love for His Son, therefore, denying the depth of the Father's love for man himself. This is the Father's love, so great as to sacrifice His beloved that He might win back us all, and receive His successful Son again into His glory once His mission of suffering and redemption was completed on Earth. ( Jn.3:16; 17:1-8,26; Heb.2:9-18, key vs.16. This Jesus, who was Christ from birth, was human, from the seed of Abraham!) This, the spirit of antichrist cannot comprehend, nor is capable of communicating to the heart of man, for the love of God does not reside within him. If attempted for deception, it is hypocritical, and is clearly transparent to the eye of the Spirit-filled believer! This is how that false spirit can be identified in man to the true believer; so do not accept a man for what he says, but what the Spirit shows you. If his "love" is conditional and selectively targeted, especially toward those who can best benefit him, while ignoring, despising, or hating altogether those who would serve him no good, this man has not the love of God in him; therefore, his spirit is false. This is how we know who is of kindred spirit, of the same family in Christ. (Mat.12:46-50; Mat.7:21-23; Rev.22:14-15. Not just having the knowledge of, but doing as well.) Consequently, it is not enough for one to say that he loves his brother, for we would never have known the love of the Father had He not shown us through the actions of His Son. Hence, love is only known through the deeds one does toward the object of one's stated love.
   
5. There are many ways to know that Christ is authentic and that if one confesses Him, he is in Him: the witness of others, a sense of one's own faith, answered prayer, an abhorrence of evil, a desire to do good, etc., but these can all be counterfeited. John has been saying to us that there is only one sure way and that is the witness of the Father in the life of the one who is being obedient to His one commandment (as He was in Christ)—loving as He loved. For His love, agape love, is incapable of being counterfeited in the life of the individual. It alone produces the telltale characteristics (I Co.13:4-8), and only then in the life of the believer who is obedient to this commandment. It is now easy to see why John (and Paul, and all the others) placed such critical emphasis upon this one idea. It is the one thing that guarantees fellowship with God, secures one in Christ, fulfills all the Law, keeps one from sin, assures one of answered prayer, edifies the entire body of Christ, rightly identifies the true family of God, and lastly and not the least important, cannot be counterfeited! God's love is authentic because Christ's life is historic, and that life was not at odds with this truth, but a demonstration of it to its fullest. They were in total agreement, one in all things ( Jn.3:16; 10:30; 12:44-50; 14:7). Therefore, He is to be trusted for eternal salvation. For in neither, God nor Christ, can be found error. Accordingly, in such a life filled with His love, there can be no room for anything that is false!