Forget the past, press on to better things

One of the saddest parts of Christian ministry is meeting people who cannot or will not move on from the past. Whether it be a refusal to forgive, the constant complaint that, 'it's not like it used to be', or the refusal to let go of old beliefs in the light of new truth.
And in the gospels we are confronted with those who could not let go of their legalistic interpretation of the Old Testament and accept that it was, all along, a signpost to the coming suffering servant Messiah. These people ended up crucifying their own Lord - the very one they claimed to serve. And yet, of course, the irony was that by rejecting Jesus, they made him their own saviour (and the saviour of the world).
In Philippians 3, Paul reflects on his own past. In one sense it was a glorious past - he was the most zealous and faithful Jew you could imagine. And yet, when confronted by the Lord Jesus he was prepared to leave behind so much that he had held dear, considering it 'rubbish' (a very polite translation!) compared with the glory of knowing Jesus Christ as Lord.
Christians have to be those who are prepared for change and prepared to change - or rather, be changed. For when we die and when Jesus returns, we shall be changed. Initially, we shall be with him, though we know little about what that will be like. But then, when he returns, this frail and mortal body changed into the likeness of his glorious resurrection body (Phil 3:20). This tent will become a permanent building (2 Cor 5:1).
But a refusal to change now, a stubborn heart set on earthly satisfaction, will result in destruction rather than future transformation (Phil 3:19). So being prepared to leave behind the old ungodly ways, and to press on to a deeper faith in Christ, new and true thinking, new and righteous behaviour is essential. This is what being a Christian is all about. This is why Christ 'took hold of us' - for constant change in this life and final, glorious transformation in the future.
So meeting people who won't move on is crushingly awful because it would seem to indicate that they are heading for destruction rather than transformation.