8 He who sins is of the devil, for the devil has sinned from the beginning. For this purpose the Son of God was manifested, that He might destroy the works of the devil. 10 In this the children of God and the children of the devil are manifest: Whoever does not practice righteousness is not of God, nor is he who does not love his brother.
Those who read 1 John carefully will notice John's use of absolute contrasts: light/darkness, know God/not know God, love his brother/hate his brother, life/death, Christ/Antichrist. Some find especially troublesome the contrast of children of God/children of the devil. Who is being called "children of the devil?" Is there any way a Christian can be "of the devil," or could that only refer to unsaved people?
Any study of the particulars of 1 John must start with the spiritual state of the readers and the purpose of the epistle. It is clear that the readers are Christians, even in the immediate context of our passages under consideration. In 3:10 it is stated that the one "not of God" does not love "his brother." Only a Christian has a spiritual brother. It is also clear that John's purpose for writing is to enhance fellowship between the readers, God, and the apostolic circle (1:3-4), not to propose tests by which the readers can know they are eternally saved (These issues are discussed in detail in GraceNotes no. 37, "Interpreting 1 John.").
If this passage is saying that those who sin and are of the devil are unsaved, then all professing Christians are unsaved, because all Christians sin. That is made clear in 1:7-10. Some translators have yielded to their theological systems to translate the present tense of the verb poieo (to do) in verse 8 as "practices sin" as if John is referring only to habitual sin (as they also translate the verb sin, hamartano, in verse 9 as "practice[s] sin" or "continue to sin." See the ESV, NASB, NET, NIV). This would demand the readers impose a subtlety of interpretation on the present tense that is not normal or readily apparent in the text (This argument applies to the present tense in 3:6, 7, and 9 which is discussed in GraceNotes no. 59, "Real Christians Don't Sin?"). The habitual interpretation of the present tense introduces troublesome subjective issues of how much sin, what kind of sin, and how often can one sin, which make the test-of-salvation interpretation a disaster to any Christian who is introspective at all.
The Scriptures remind us that Christians sin and are even capable of murder (James 4:2; 1 Peter 4:15).
By his use of contrasts, John is distinguishing two different sources for Christian behavior. He did this in 3:9 where the believer in his regenerate person cannot sin because Jesus Christ cannot sin. That means sin must come from elsewhere, which is ultimately the devil who "has sinned from the beginning" (3:8). It was the devil who deceived man into sin causing man to have a sin nature. Sin is contrary to Jesus Christ's purpose which was to destroy the devil's works. Satan and Christ are totally at odds in their purposes and characters. When a believer does right, he manifests his God-given divine nature (3:9), but when he sins, he manifests his Satan-inspired sin nature (3:8, 10). The word "children" (tekna) in reference to God or the devil is not used biologically as in a genetic relationship, but is used for those who have characteristics derived from another person, that is, a kind or class of persons (It is used this way in Matt. 11:19/Luke 7:35; Gal. 4:31; Eph. 2:3; 5:8; 1 Peter 3:6.). John is simply noting the ultimate origin of a believer's actions.
The example of Cain murdering Abel in verse 12 is a physical illustration of this spiritual truth. It is not a statement about whether Cain was saved or not; it only shows that Cain's action was inspired by his envy of Abel which was ultimately inspired by the devil (who "was a murderer from the beginning;" John 8:44). Similarly, when Jesus said to Peter, "Get behind Me, Satan!" (Matt. 16:23), He was revealing the source of Peter's rebuke of Christ by which Peter demonstrated that he was representing Satan's purpose, not God's. The Apostle Paul wrote that a believer can be taken captive by Satan to do his will (2 Tim. 2:26; Compare Acts 5:3). James 3:15- 17 shows that there are two sources for a believer's choices, one that is demonic (Satan's) and one that is from above (God's). The New Testament perspective is clear: at times believers can do the work of the devil.
Christians sin, and when they do, their actions must be sourced in the devil's influence, not God's. Sin does not prove that one is not a Christian, only that he is representing the devil and his will. A Christian shows himself to be a child of the devil or "of the devil" by doing the devil's will and works. It is futile and damaging to test one's salvation or try to prove one is unsaved by the reality of sin in one's life. The only test of salvation given in 1 John is whether one has believed in and possesses Jesus Christ, the Son of God (5:1, 11-13). That is the only test that keeps the gospel of grace totally free from human merit or performance.