Marks of True Christian Fellowship: 1 John 2:1-6. 
Christianity is not a religion but a relationship.  John shows that those who believe in Jesus Christ demonstrate their faith by their obedience to Christ's command. They will have a desire to do what pleases the Lord.  If someone claims “I know the Lord” and lives in sin, he is a liar. Those who have true knowledge of God obey Him and the love of God is perfected in them. The secret of obedience is the love for God. They live just as the Lord Jesus lived in the power of the Holy Spirit. Those who do not obey the Word of God are blind and do not know where they are going (2 Peter 1:9). John addresses believers "Little children".  It shows his endearment toward believers. He assumes that believers can be influenced by false teachers to sin. But if anyone does sin, we have an advocate with the Father, Jesus Christ the righteous.  The basis of their salvation is the atoning sacrifice of Christ.  They are free from God's wrath because Jesus already endured it on their behalf.  God's wrath is his settled attitude toward all that is unholy. Christ is the propitiation for our sins. The word propitiation refers to Christ's sacrifice that satisfies the wrath of God against sinners. Only those who believe in Christ are rescued from the wrath of God (5:12). Those who truly believe demonstrate their faith by obedience. They obey the Lord because they love Him. This is a mark of genuine believers who are transformed by the Holy Spirit. Our obedience is the evidence of our salvation. Disobedience weakens our sense of assurance of salvation.   Professing Christians receive the grace of God in vain.  Are you saved by believing in the Lord Jesus Christ?  

Another mark of true believer is the love for the brethren. 1 John 2:7-11.  
John now highlights Christ's command to love one another (2:9-11). Being a Christian involves more than words but means demonstrating sacrificial love to others. This is not a new command.  In a believer's life, love and light cannot be separated. We love God because He first loved us (1 John 4:19). John is known as the apostle of love. Once he was known as the son of thunder, which simply means hot temper. He wanted to call down fire to destroy the villagers who did not receive Christ (Luke 9:53, 54). When John was transformed by the love of Christ, he was willing to call believers “beloved” (2:7).  God’s love is the agape love which carries the idea of self sacrificing love. The highest expression of divine love is revealed on the cross (Romans 5:8). There is no love like the love of a dying Savior.  There is a friendship love (Philia) and a sensual love (Eros). The word for sensual love is not used in the New Testament.  In the Old Testament, God commanded to love neighbor (Deuteronomy  6:5; Leviticus 19:18; Mark 12:28-34).  Love is the fulfillment of God’s Law (Romans 13:8-10).  It is the proof that we have passed from death unto life (1 John 3:14). Jesus said to the disciples "You are my my disciples, if you have love for one another" (John 13:35). The Holy Spirit sheds abroad God’s love in our hearts (Romans 5:5).  Jesus loved the world with sacrificial love (John 3:16).  He loved the lowest and the highest, the oldest and the youngest.  He comforted the sick and the hungry, loved the foes and the friends.  Jesus said “There is no greater love than to lay down one’s life for his friends” (John 15: 13). A person who claims to be saved and continues to hate his brother is still in darkness. It is impossible to be in fellowship with God and out of fellowship with fellow brothers at the same time. When we practice Christian love, there is no cause for stumbling. The best way not to become a stumbling stone is to love the brethren. Grudges and bitterness are poison that affects our relationship with God. Harboring such sins can lead to other sins. Love thinks no evil, covers multitudes of sins and expels all fear (1 Corinthians 13:5; 1 Peter 4: 8; 1 John 4:18).  Love is the fruit of the Spirit (Galatians 5:22, 23).  If you are having trouble loving others, look at Jesus. He reached out to the unlovable (Luke 19:1-10).  Jesus showed patience toward those who questioned him (John 20:24-28).  He gave a second chance to Peter who had let him down (John 21:15-19). We are not only commanded to love but also enabled to love one another. Those who do not love the brethren are spiritually blind and barren.

Hindrances to Fellowship: 1 John 2:12-17.  
John speaks to the various levels of spiritual maturity in his epistle.  He calls believers “Little children, fathers and young men". These are word pictures used to show the various levels of spiritual maturity among believers. They enjoy forgiveness, fellowship and victory in Christ. In one sense all believers are little children to God (1 John 3:1).  John also assures them of his confidence in them that their sins have been forgiven. Little children are babies in the faith. Young men are strong in the faith and they overcome Satan.  Fathers are spiritually mature in knowledge and experience. John addresses the hindrances to fellowship here.  Christians are in the world but not of this world. They are warned not to love the world system. Satan is the prince of this world (John 12:31). World refers to the invisible spiritual system under Satan that opposes the work of God.  Unsaved are the sons of this world (Luke 16:8).  World hates believers because they don't belong to this world (John 15:18). The believer’s citizenship is in heaven and they should not love the world (Philippians 3:20). They should not love the world or the things in the world.   If anyone loves the world, the love of the Father is not in him (1 John 2:15). We are called to turn away from worldliness and materialism. One who continues to love the world is not of the Father (1 John 2:16). This world is passing away but whoever does the will of God abides forever (1 John 2:17).

Warning against False Teachers: 1 John 2:18-29.  
John presents another proof of genuine faith. It is not enough to be sincere in loving God but we must also know the truth.  Believers were warned against false teachers by Christ and apostles (Matthew 7:15; 1 Timothy 4:1). False teachers are influenced by the spirit of Satan. Satan is the father of all lies and counterfeit of truth (John 8:44). False teachers deny the deity of Jesus and work against Him. They associate with the believers but they are not born of God (Jude 19). If they were true believers, they would not have left the church (2:19). One of the proofs of true Christian is a desire to be in fellowship with the people of God. True believers agree on the doctrines of the Bible and they are in fellowship with other believers. False teachers deceive believers by claiming that they have special anointing to know the truth. John says "If false teachers had the anointing they would not have denied the deity of Jesus".  True disciples follow the footstep of Jesus. They take time to study the Word, spend time in prayer and find time to fellowship with God’s people. John encourages the readers that they have the anointing from the Holy Spirit to know the truth (1 John 2:27; Romans 8:9; 2 Corinthians 1:21; John 14:26).  False teachers deny that Jesus is Christ. In this context the false teachers are called "antichrists" (1 John 2:18). They do not know the truth because they do not have the Holy Spirit (Jude:19).  Believers have no need to listen from the false teachers because they have the Holy Spirit to guide them in all truth. Those who deny the Son deny the Father.  Jesus said “I and the Father are one” (John 10:30).  He who honors not the Son honors not the Father who has sent Him (John 5:23).  Whoever has the Son has eternal life (1 John 5:11-12). Those who are saved should continue in fellowship and live in anticipation of Christ’s return to receive him with confidence (2:28).  Pray that our devotion to Christ may grow more in the knowledge of him. The greatest identifying mark of a disciple is that his walk resembles Jesus his Master.