You have an anointing from the Holy One, and you know the truth.
The answer to our query, "What is the unction, or anointing?" is expressed in the context. Consider the following: (1) Everyone had received the anointing (Verse 20). It was not just for a select few, as in the case of the special gift of knowledge (1 Corinthians 12:4-11; 1 Corinthians 12:28). (2) This anointing gives knowledge. John says, "And ye know all things." Other versions say, "You all know." (3) Look at verse 24: "Let that therefore abide in you, which ye have heard from the beginning. If that which ye have heard from the beginning shall remain in you, ye also shall continue in the Son, and in the Father." What is to abide in these Christians? John says, "that which ye have heard," that is, the message which had been preached from the beginning of their Christian experience.