Philippians 1:21

We have here an account of the life and death of blessed Paul: his life was Christ, and his death was gain. Observe, 1. It is the undoubted character of every good Christian that to him to live is Christ. The glory of Christ ought to be the end of our life, the grace of Christ the principle of our life, and the word of Christ the rule of it. The Christian life is derived from Christ, and directed to him. He is the principle, rule, and end of it. 2. All those to whom to live is Christ to them to die will be gain: it is great gain, a present gain, everlasting gain. Death is a great loss to a carnal worldly man; for he loses all his comforts and all his hopes: but to a good Christian it is gain, for it is the end of all his weakness and misery and the perfection of his comforts and accomplishment of his hopes; it delivers him from all the evils of life, and brings him to the possession of the chief good. Or, To me to die is gain; that is, "to the gospel as well as to myself, which will receive a further confirmation by the seal of my blood, as it had before by the labours of my life." So Christ would be magnified by his death, Phi 1:20.

— Matthew Henry's Concise Commentary (public domain)

For--in either event (Phi 1:20) I must be the gainer, "For to me," &c. to live is Christ--whatever life, time, and strength, I have, is Christ's; Christ is the sole object for which I live (Gal 2:20). to die is gain--not the act of dying, but as the Greek ("to have died") expresses, the state after death. Besides the glorification of Christ by my death, which is my primary object (Phi 1:20), the change of state caused by death, so far from being a matter of shame (Phi 1:20) or loss, as my enemies suppose, will be a positive "gain" to me.

— Jamieson-Fausset-Brown Commentary (public domain)