Mark 2:22 “And no one puts new wine into old wineskins. If he does, the wine will burst the skins—and the wine is destroyed, and so are the skins. But new wine is for fresh wineskins.”
Wine is always symbolizing doctrine of faith. Jesus is teaching that the person must be born again and thus made new to receive the doctrine of faith properly.
If the person is still clinging to his former way of life – ie. paganism – then the person will be mislead and the gospel that he teaches will be erroneous.