How can John not know what we will be like in I John 3:2?

Answer:

"Behold what manner of love the Father has bestowed on us, that we should be called children of God! Therefore the world does not know us, because it did not know Him. Beloved, now we are children of God; and it has not yet been revealed what we shall be, but we know that when He is revealed, we shall be like Him, for we shall see Him as He is. And everyone who has this hope in Him purifies himself, just as He is pure" (I John 3:1-3).

No one on earth has experienced what life in heaven is like. "No one has ascended to heaven but He who came down from heaven, that is, the Son of Man who is in heaven" (John 3:13). No one here has experienced what life is like in a spiritual body. "Who will transform our lowly body that it may be conformed to His glorious body, according to the working by which He is able even to subdue all things to Himself" (Philippians 3:21). And none of us have experienced a life where we are not constantly battling sin. We know we will become spiritual beings like God, but exactly what that means, we don't know. We know the generalities, but we don't know it first hand.