John Calvin Commentary 2 Corinthians 5:4

"For indeed we that are in this tabernacle do groan, being burdened; not for that we would be unclothed, but that we would be clothed upon, that what is mortal may be swallowed up of life." — 2 Corinthians 5:4 (ASV)

We groan, being burdened, because we desire not to be unclothed. The wicked, too, groan because they are not content with their present condition; but afterwards an opposite disposition prevails, that is, a clinging to life, so that they view death with horror, and do not feel the long continuance of this mortal life to be a burden. The groaning of believers, on the other hand, arises from this: they know that they are here in a state of exile from their native land, and they know that they are here shut up in the body as in a prison. Hence they feel this life to be a burden, because in it they cannot enjoy true and perfect blessedness, because they cannot escape from the bondage of sin except by death, and hence they aspire to be elsewhere.

However, since it is natural for all animals to desire existence, how can it be that believers are willing to cease to exist? The Apostle solves this question when he says that believers do not desire death for the sake of losing anything, but because they anticipate a better life. At the same time, the words express more than this. For he admits that we naturally have an aversion to leaving this life, considered in itself, just as no one willingly allows himself to be stripped of his garments. Afterwards, however, he adds that the natural horror of death is overcome by confidence, just as an individual will, without any reluctance, throw away a coarse, dirty, threadbare, and, in a word, tattered garment, expecting to be arrayed in an elegant, handsome, new, and durable one.

Further, he explains the metaphor by saying:

that what is mortal may be destroyed by life. For as flesh and blood cannot inherit the kingdom of God (1 Corinthians 15:50),

it is necessary that what is corruptible in our nature should perish, in order that we may be thoroughly renewed, and restored to a state of perfection. On this account, our body is called a prison in which we are confined.