"And every man that hath this hope in him purifieth himself, even as He is pure." (1 John 3:3)
So John's continuing on from his initial thought in the previous verse: "now we are the sons of God, and it doth not yet appear what we shall be: but we know that, when He shall appear..."
It's about staying focused. I don't really see, for all the two-thousand years of history between the writings of John and now, an answer for what he spoke of in this. It still doesn't appear. As a quick aside, I find the current bumper crop of post-death-experience writings to be both a boon and also a potential distraction. But intriguing all around. Firstly, "try the spirits whether they are of God" (1 John 4:1). Secondly, and the ones that pass muster intrigue me because how can all these people be experiencing Heaven and have correlating stories about a God who is loving and who, all He wants is to see you there with Him, be wrong? And forgive me for begging the question. Staying focused, yes.
The whole reason I cite the top verse is because it's a one line prescription for "purification". Something that can be so hard to come back around to. John centers it on the deliberate meditation upon that which God is preparing for us. Jesus says to His disciples: "And if I go and prepare a place for you, I will come again" (John 14:2a). Jesus has been busy doing God knows what and we are asked by John in his first letter to keep it in mind. Christmas is coming. So is this.
"And whither I go ye know, and the way ye know." (John 14:4)
Heaven, and the beauties that await, are worth the wait. Worth all the pain and suffering and boredom we face with our day to day lives. But in the interest of surprises and all that, I'm hesitant to read firsthand accounts. Because of whatever little piece of Heaven the Lord has allowed me to realize in the here and now.