But ye have an unction from the Holy One, and ye know all things.
But the anointing which ye have received of him abideth in you, and ye need not that any man teach you: but as the same anointing teacheth you of all things, and is truth, and is no lie, and even as it hath taught you, ye shall abide in him. (1 John 2:20, 27 KJV)
I have puzzled over verse 20 for a few days. Then I read verse 27 and it helped clarify what verse 20 was about.
Do these verses mean that we know everything? Of course not! We would be God if we did. Do they mean we do not need preachers and Bible teachers? No, because that would be a contradiction of the whole book Romans and the Pastoral epistles.
I believe the whole point of these verses is that the Holy Spirit teaches how we should live and convicts us of sin in our lives through the reading and study of the Word so that we repent and walk in the Light. This is how we come to abide in Him. We "know all things" that matter in relation to walking in the Light and abiding in Him.