"Dear friends, now we are children of God, and what we will be has not yet been made known. But we know that when he appears, we shall be like him, for we shall see him as he is."
It is when we look upon Christ "as he is" that "we shall be like him". God has created us to be in his image, that is the human image of Christ, so that we might always reflect God's glory. Like mirrors, we shine only that which we behold. So if our eyes are turned towards Christ, will they not reflect his glory as the risen Son of God exalted to the right hand of the Father?
We are children of God. We are mere children who have not yet grown into heavenly adults with all the rights and privileges of kingdom citizenship. We cannot yet vote in the Kingdom. Our adult bodies may fool us into thinking we are grown, they are but flesh and not spirit. But he who looks upon Christ shall be mature in spirit, perfect in every way and as verse 3 puts it, "pure as he is pure".
But rest assured, my beloved, you will be men and women of God by the power of simply beholding the risen Christ slain from the foundation of the world. As you draw nearer to Christ your vision of him will only become clearer until the great and fearful day of his coming when you will see him as he truly is: The risen messiah, broken for you.