John did not speak of a singular antichrist. He identified that as hearsay, and instead enlightened his "little children" that there were already many antichrists.The opposite is true. All should be aware that many before you have denied a single person to fulfill this role, only to be proven wrong by scripture.
John spoke of a singular and particular antichrist who was to yet come:
1 John 2:18 Little children, it is the last time: and as ye have heard that antichrist shall come, even now are there many antichrists; whereby we know that it is the last time.
He says "now" there were many antichrists but does specifically speak of one certain antichrist (singular) that was yet to come (future arrival) which means John believed there was a future Antichrist. This is known as "the Antichrist" which is a specific Antichrist (also the last one) among all those who are antichrists.
John does not say how far into the future that this specific Antichrist would come but he did speak in the future tense.
Paul spoke of this same singular evil person using different names:
2 Thessalonians 2:3 Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;
2 Thessalonians 2:4 Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.
2 Thessalonians 2:5 Remember ye not, that, when I was yet with you, I told you these things?
2 Thessalonians 2:6 And now ye know what withholdeth that he might be revealed in his time.
2 Thessalonians 2:7 For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way.
2 Thessalonians 2:8 And then shall that Wicked be revealed, whom the Lord shall consume with the spirit of his mouth, and shall destroy with the brightness of his coming:
2Th 2:9 Even him, whose coming is after the working of Satan with all power and signs and lying wonders,
John writes of this same singular antichrist person calling him different names/terms in the book of Revelation, the false prophet and the (second) beast.
Rev 13:11 And I beheld another beast coming up out of the earth; and he had two horns like a lamb, and he spake as a dragon.
Rev 13:12 And he exerciseth all the power of the first beast before him, and causeth the earth and them which dwell therein to worship the first beast, whose deadly wound was healed.
Rev 13:13 And he doeth great wonders, so that he maketh fire come down from heaven on the earth in the sight of men,
Rev 13:14 And deceiveth them that dwell on the earth by the means of those miracles which he had power to do in the sight of the beast;
Rev 19:20 And the beast was taken, and with him the false prophet that wrought miracles before him, with which he deceived them that had received the mark of the beast, and them that worshipped his image. These both were cast alive into a lake of fire burning with brimstone.
What name or term used to describe this singular evil person is irrelevant because many names are used whether Antichrist, false prophet, (second) beast, man of sin, that Wicked, the son of perdition, the little horn, abomination of desolation and so on. It's all speaking of the same individual who will reign the world during the great tribulation and claim to be God and perform miracles and wonders.
The concept of a future singular "antichrist" person is biblical. Who this person is has been debated very much, but denying that there is a future singular antichrist denies what scripture tells us.
John is the only NT author to use the word "antichrist(s)".
A future singular antichrist is a fabrication spawned in the counter-Reformation of the 16th-century apostate papacy.