Matthew 9:16 & 17
"'No one sews a patch of unshrunk cloth on an old garment, for the patch will pull away from the garment, making the tear worse. Neither do people pour new wine into old wineskins. If they do, the skins will burst; the wine will run out and the wineskins will be ruined. No, they pour new wine into new wineskins, and both are preserved.'" NIV translation
Jesus continues to answer the question of John's disciples with a lesson on religion and grace. They would have been familiar with the concept of mending clothes and of transporting wine and so Jesus used those examples to show them that religion and grace cannot be mixed. We are reminded that John's disciples would have been Jews and, to them, nothing was legally established without two witnesses (examples).
In the first example, if you sew a new piece of cloth over a hole in an old piece of cloth, the new piece will shrink when it is washed while the old will not. This will cause the new cloth to pull on the old and make the rip worse.
In the second example, a new wine skin has the ability to stretch while an old one would already be stretched to its limit. This stretching was necessary as the grape juice would age (ferment) and give off gases which would cause the pressure to build in the skin. If the skin could not stretch, it would break and be ruined as the wine spilled onto the ground.
So, what does all of that have to do with the struggle between religion and grace? Jesus was (and is) offering people a "new" life and this is not compatible with the old life of trying to achieve righteousness through our own actions. It is a life of freedom and, if you understand and accept that gift (grace), you will not feel the need to follow religious rituals. With the wineskin example, we see that the destruction of religion might take a little bit of time (such as the time it takes the grape juice to ferment) but the result is the same. As we grow in our faith, the religious trappings will fall away and we will be left with simply trusting in the finished work of Jesus on the cross.