For more context (RSVCE version):
"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. 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 be plain that they all are not of us. 20 But you have been anointed by the Holy One, and you all know. 21 I write to you, not because you do not know the truth, but because you know it, and know that no lie is of the truth. 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. 23 No one who denies the Son has the Father. He who confesses the Son has the Father also. 24 Let what you heard from the beginning abide in you. If what you heard from the beginning abides in you, then you will abide in the Son and in the Father. 25 And this is what he has promised us, eternal life.
26 I write this to you about those who would deceive you; 27 but the anointing which you received from him abides in you, and you have no need that any one should teach you; as his anointing teaches you about everything, and is true, and is no lie, just as it has taught you, abide in him.
28 And now, little children, abide in him, so that when he appears we may have confidence and not shrink from him in shame at his coming. 29 If you know that he is righteous, you may be sure that every one who does right is born of him."
I don’t know for certain, and I also haven’t studied this passage in any depth. BUT, I would never take one passage alone, especially if someone is using a passage to argue against something we do in the Catholic Church…scripture will always support the Church…we often forget it is OUR book, we “made” it (compiled it). Right? The Church is not based on scripture, but rather Scripture came from the Church…and so be confident it will always be on our side.
Taking it in context, What it looks like to me, is that John is giving a warning to be wary of false-prophets and false-teachers (“antichrists”). It looks like he is saying that since you know the truth (i.e. you know know Jesus…Jesus is Truth) you know enough to be able to spot the antichrists and their false teachings. You know what comes from Christ because you know Christ, having been anointed by him.
…Just my thoughts. I’m sure there will be better scholars here who can give better input.