1 john 2:18-27

v.18- "children, it is the last hour, and as you have heard that antichrist is coming, so now many antichrists have come. therefore we know that it is the last hour."the last hour refers to the time between Jesus' resurrection and His second coming. there are many antichrists/ false teachers/ false prophets (2 thessalonians 2:1-10) who Jesus warned of (matthew 24:5, 24)- most will appear genuine, but ultimately, what defines an antichrist is his denial of Jesus as Christ (v.22). 
v.19- "they went out from us, but they were not of us; for if they had been of us, they would have continued with us. but they went out, that it might become plain that they all are not of us."
john addresses those who had recently left the church by saying that they were never genuine believers (acts 20:30). this implies that once a person is truly saved, he will never abandon Christ nor will he lose his salvation (jude 24, john 17:12). God made it plain (1 corinthians 11:19) that those who left were not His so that the church would be sanctified. church is for believers and therefore, the church should not try to appeal to the world. 
v.20~21- "but you have been anointed by the Holy One, and you all have knowledge. i write to you not because you do not know the truth, but because you know it, and because no lie is of the truth."
those who have been transformed by the Holy Spirit live by the gospel (2 corinthians 1:21). john writes this letter to remind Christians to go back to the knowledge they have been given (john 14:26) and to abide in the teachings of Christ (v.27). 
v.22- "who is the liar but he who denies that Jesus is the Christ? this is the antichrist, he who denies the Father and the Son."
it is obvious to believers when the antichrists deny Jesus as the Christ, (2 john 7) but the devil will try to deceive the elect by disguising himself and by twisting scriptures. 
v.23- "no one who denies the Son has the Father. whoever confesses the Son has the Father also."
since the Son and the Father are both God, no one can have one and not the other. Jesus is the only way to the Father (john 14:6).
v.24- "let what you heard from the beginning abide in you. if what you heard from the beginning abides in you, then you too will abide in the Son and in the Father."
the gospel message which we heard calls us to love God and those who love Him will obey His commandments (2 john 6, john 14:23). 
v.25- "and this is the promise that He made to us- eternal life." 
God will keep those whom He predestined (john 17:2) to be saved, because He is faithful to His promises. therefore, we who have been saved can stand confidently before God on the day of judgement.
v.26~27- "i write these things to you about those who are trying to deceive you. but the anointing that you received from Him abides in you, and you have no need that anyone should teach you. but as His anointing teaches you about everything, and is true, and is no lie- just as it has taught you, abide in Him." 
in this context, john means that believers have all the knowledge they need from the gospel (jeremiah 31:34). this implies that knowledge/ education which is not based on true biblical doctrine is not necessary. hence, we should study the bible to discern what is the truth (john 14:17) and what is a lie from the devil and his antichrists.