John Calvin on Matthew 5:4

4. Happy are they that mourn. This statement is closely connected with the preceding one, and is a sort of appendage or confirmation of it. The ordinary belief is, that calamities render a man unhappy. This arises from the consideration, that they constantly bring along with them mourning and grief. Now, nothing is supposed to be more inconsistent with happiness than mourning. But Christ does not merely affirm that mourners are not unhappy. He shows, that their very mourning contributes to a happy life, by preparing them to receive eternal joy, and by furnishing them with excitements to seek true comfort in God alone. Accordingly, Paul says,

“We glory in tribulations also knowing that tribulation produces patience, and patience experience, and experience hope: and hope maketh not ashamed,” (Romans 5:3-5.)