The sorrow of disciples, not the sorrow of the world
Jesus did not explain to His disciples in more detail what kind of sorrow He meant, and this is often how it is in the Scriptures. Spiritual matters are interpreted with spiritual words, and it is up to each individual to understand it rightly, in their own circumstances. But the whole sermon is addressed to disciples, so He is referring to the grief and sorrow of disciples, not the sorrow of the world, which produces death. 2 Cor. 7:10. The world sorrows over a lack of money, food, clothes, houses, honor, power, reputation, and so on.
Those who mourn are those who are no longer on the same wavelength as the world, neither are they willing to conform to it. They mourn over the world and its fate. The world fantasizes about progress, power, and the future; the disciples know that the kingdom of heaven is coming and that the world is not ready to meet it. They live as strangers in the world, with power from Him, the One the world found so alien that they crucified Him. They find their comfort in the cross; they find their rightful home with the crucified Christ, both here in time and in eternity.
They bear others in their hearts, just as they are. Their need is: How can I make you understand that you do not have victory; how shall I help you out of your misery? This need produces sorrow, but also fervent prayer and cries of need to God, and He is able to intervene in His time. What a comfort!
Sorrow over the lack of divine life
Above all, their sorrow is that they still have so little of what is in God. So little power, so little of all the virtues of Christ, that they serve and give their lives too little, are not steadfast enough, lack wisdom, deal with matters the wrong way, and so on. Those who with their whole heart want to do good find that evil is present with them. Rom. 7:21. This produces a deep sorrow: “O wretched man that I am!” V. 24.
The thing is to always sorrow for something better, as well as for more of the good we have already partaken of. Such sorrow has everlasting consequences; it produces diligence, clearing of ourselves, indignation, fear, vehement desire, zeal, and vindication (2 Cor. 7:11); it is a driving force to making progress and growth in all the virtues. For a disciple, this is the very best comfort.
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References
Johan Oscar Smith. Summary of a Bible Study Held in Oslo, December 28, 1921. Skj. Sk. Dec. 1922.
Elias Aslaksen. Bible studies in the Gospel of Matthew. Published as a series in Skjulte Skatter between 1936 and 1941.
Dietrich Bonhoeffer, The Cost of Discipleship.
