From Revelation by Peter S. Williamson commenting on 7:15-17:
An end to suffering. The vision in Rev 7:9–17 of the eternal life promised us is extraordinarily comforting. Here is no empty promise that faithful Christians will be spared trial and suffering, a way of thinking that the life of Jesus and all of Christian history contradict. Rather, John foresees a countless multitude passing through the great tribulation of this age before Christ returns and standing victorious before God and the Lamb, wearing white robes, and waving palms in celebration.
This vision offers a partial yet helpful answer to an age-old question: How can a God who is good and all-powerful allow suffering to afflict the just? The vision reveals that all such affliction is time limited. There will be an irreversible end to the suffering of those who belong to God. Those who have “washed their robes and made them white in the blood the Lamb”—the Lamb who fully shared in the suffering of this world to the point of being slain—are destined to eternal joy in God’s presence, where there will be no more hunger, thirst, oppressive heat, or any other evil. The Lamb will be their shepherd and quench their thirst with life-giving water. God their Father will heal their wounds and “wipe away every tear from their eyes.”
For all the saints. The Church reads the two visions of this chapter on the Solemnity of All Saints, a feast that celebrates the sanctity of all God’s faithful people who have gone on before us, not just the saints who have been canonized by the Church. The Lord calls every Christian to holiness, in every state of life. Hebrews 12:14 exhorts us, “Strive for . . . that holiness without which no one will see the Lord.” It is a simple fact that only saints—holy people—reach heaven. Christ qualifies us to live in God’s presence: he makes us holy through his death and resurrection, conveying the benefits to us through the sacraments. Nevertheless, we must “strive” to do our part as well through ongoing repentance, prayer, reading Scripture, and loving our neighbor as ourselves. Our part entails daily dying to self and surrendering ourselves completely to God. This process must be completed before we see the face of God. Why not begin in earnest now?