In ancient times, wine should quickly be stored in containers called wineskins or else it would quickly turn into vinegar. Such wineskins have to be new, and their quality of the best kind, or else it would run the risk of bursting, as it cannot hold wine.
As Christ's new creation and having a new spirit, what part of our old selves are we still clinging to? What part of the former "you" is holding you back in order to becoming the new creation that Christ called you to be?