"I do not write to you because you do not know the truth, but because you do know it and because no lie comes from the truth." NIV translation
We have the Holy Spirit living inside us as Christians and with that comes knowledge from God. We are able to understand God's Word because of the power of that Spirit. John is telling us that here and reminding us that lies and the truth do not mix. Have you ever heard someone say something and claim that it is from God but you just know that it cannot be? That is the Holy Spirit guiding you into the truth contained in God's Word.
"Who is the liar? It is the man who denies that Jesus is the Christ. Such a man is the antichrist - he denies the Father and the Son. No one who denies the Son has the Father; whoever acknowledges the Son has the Father also." NIV translation
God is not a supermarket and you don't get to choose what part of Him you want to accept and what part you reject. There are many in the world who accept the fact that God has created everything but they reject Jesus as the way to a right relationship with Him.
John tells us here that people like this are the antichrist. This simply means that they oppose the work of Jesus Christ and the chief of the opposition is Satan who is also called the father of lies.
"See that what you have heard from the beginning remains in you. If it does, you also will remain in the Son and in the Father." NIV translation
John is urging believers here to hold fast to the gospel of Jesus Christ and not let anyone pull them away through false teaching. In verse 19, he told us that some had appeared to have lost their faith/salvation but, in fact, they never had it. We have a modern example of this as there are many who would have called themselves Christians but now have turned to Islam.
"And this is what he promised us - even eternal life." NIV translation
There is nothing in the world like the retirement package for the Christian and this is what Jesus promised. When our Father gives the word, Jesus will be back to get us to take us to be with Him forever. Then, the good life begins with the new heaven and the new earth (see Revelation 21 & 22).