Yes, I sin. But that does not separate me from God. If it did, I would not be convicted of that sin and I would never repent. No one is separated from God as long as they live and breathe. But when that ceases, unless we have put on immortality, we are truly separated from God.
I would like to expand on something. Being separated from God can be seen as not "experiencing" God's love or presence. When we sin, God's love and presence doesn't "go away". However, guilt, fear, and shame can cause us not to be able to feel or experience God's love and presence. Through confession and repentance we get feelings of guilt, fear, and shame out of the way and we begin to experience the love and presence of the divine that was always there and never left.
For example, Adam and Eve. After they sinned they realized their nakedness and made aprons of figs (shame). When they heard God coming into the garden, they fled and hid themselves in the trees (fear). God then asked, "Adam, where art thou?" Now, we know that an all knowing God isn't asking the question for His own benefit. He was asking it for Adam's. He wanted Adam to consider the fact that he had attempted to separate himself from God, his Father. God's question was to cause Adam to think, "Where am I? What am I doing?" When God confronted them Adam blamed God, Eve blamed the serpent. They wanted to pass the buck (guilt). Did God ever stop loving them? No. Did God depart from them? No. But THEY hid themselves from God and couldn't experience Him through their guilt, fear, and shame. Separation. We most often cut ourselves off from God emotionally because of our sin. God covered their nakedness Himself and therefore eliminated the barriers to His love and mercy. Just as He did our own upon the cross.
So, can we be separated from God while we live? Yes and no. It depends upon what one means by being "separated" from God.
But when that ceases, unless we have put on immortality, we are truly separated from God
What makes you think that one can be separated from God even in death? Don't you realize that even if we make our bed in hell (however one wishes to define it), God is there? One cannot escape God. God also "loses" no one. And, many have noted that if Jesus has the "keys to death and hell", He holds the power to open and close the gates of death and hell at will. This would mean that even death itself doesn't separate us from God's power to purify and save.
The doctrine of Hell is interesting in that many devout Christians interpret it differently. Some Christians say that Hell is a place of eternal torments where souls will languish in agony for all eternity, for ever and ever, without end. Some Christians say that Hell is a place where God is to destroy the body and soul of the wicked, vaporizing both them and their evil forever. And yet other Christians view Hell as a terrible place of spiritual judgment and purification. A refiner's fire wherein all sin, self, and rebellion are burned away as dross. Where the soul that rejected, or never knew Christ, is purified. And when that purification is done, every man will be delivered by the one who holds the keys to death and hell, in their own order. Eventually every human soul will bow their knees and confess that Jesus Christ is Lord and Saviour. God's plan to save mankind will have succeeded 100%. And all will rejoice, realizing that it was only through the blood of the Lamb of God that God was justified in saving all. For without Christ, ALL would have perished.
Now, I don't "know" which doctrine is true. However, as individuals we have to evaluate a doctrine and compare it both to Scripture and the Spirit and nature of God. Which doctrine best exemplifies God's nature? Which doctrine brings God the most glory?