Those who mourn are the people who felt sad about Israel's spiritual and national condition (see Matthew 23:37–39; Isaiah 61:2–3; Joel 1:8–13; 2:12–13, 18–19; Romans 9:1–5; 1 Corinthians 5:2; James 4:9). It may also include to personal sorrow, whether from sin or tragedy (Mark 16:10; Revelation 18:11, 19).
will be comforted: The passive voice shows respect for God's holy name. It could also mean God will comfort them (see also Matthew 5:6–7, 9). Comfort will come when God fulfills all his promises (Isaiah 40:1–2; 61:2–3).