If you look up the word agnostic in the dictionary you find that the etymology of the word is Greek agnostos which means unknown, or unknowable. If I am fully honest, I don’t know and in this lifetime thing that afterlife is by definition unknowable. Believing about an afterlife really has nothing to do with whether or not there is one or what it is ultimately like.
My theology, as it has developed over the years, does not follow the popular theology that has risen up recently that places such an emphasis on aferlife. I, like Borg, would suggest that is one of the negative characteristics of modern Christianity. By putting such an emphasis on afterlife, we somewhat mute our responsibility in this life.
When I was in college, I wrote my senior thesis on John Wesley and his affects on the English Working Class Movement. My argument was that John Wesley, unintentionally held the English workers from striving for improved working conditions because of the promised reward in the afterlife. Karl Marx argued basically the same thing.
Borg offers some other reasons why he is agnostic about the afterlife. I think he is right on and I will just quote the rest of the article (I wish I knew where I got it from).
When the afterlife is emphasized, it almost inevitable that Christianity becomes a religion of requirements and rewards. If there is a blessed afterlife, it seems unfair to most people that everyone gets one, regardless of how they have lived. So there must be something that differentiates those who get to go to heaven from those who don’t – and that something must be something we do, either believing or behaving or some combination of both. And this counters the central Christian claim that salvation is by grace, not by meeting requirements.
Another problem: the division between those who "measure up" and those who don’t leads to further distinctions: between the righteous and the unrighteous, the saved and the unsaved.
Another problem: an emphasis on the afterlife focuses our attention on the next world rather than on this world. Most of the Bible, on the other hand, focuses our attention on our lives in this world and the transformation of this world. At the heart of the Lord’s Prayer is the petition for the coming of God’s kingdom on earth: your kingdom come on earth, as it already is in heaven. There is nothing in the Lord’s Prayer asking that God take us to heaven when we die.
As yet another reason for my agnosticism about an afterlife: does it involve the survival of personal identity and reunion with those we have known in this life? Are family reunions part of the afterlife? For some people, this is much to be desired, for family has been the primary source of love and joy in this life. But for perhaps an equally large number of people, family has been the primary source of pain and unhappiness. So, are we going to be with those people forever?
What I do affirm about what happens after death is very simple: when we die, we do not die into nothingness, but we die into God. In the words of the apostle Paul, we live unto the Lord and we die unto the Lord. So whether we live or die, we are the Lord’s.
For me, that is enough. My not knowing anything more does not bother me at all.
And I am very wary when the Christian gospel becomes a message about the afterlife. I am convinced that it invariably leads to distortion. This is not the Christian gospel.