From the Treasury of Daily Prayer:

This feast is the most comprehensive of the days of commemoration, encompassing the entire scope of the great cloud of witnesses with which we are surrounded. It holds before the eyes of faith that great multitude which no man can number: all the saints of God in Christ–from every nation, race, culture, and language–who have come “out of the great tribulation…who have washed their robes and made them white in the blood of the Lamb.” As such, it sets before us the full height and depth and breadth and length of our dear Lord’s gracious salvation. It shares with Easter a celebration of the resurrection, since all those who have died with Christ Jesus have also been raised with Him. It shares with Pentecost a celebration of the ingathering of the entire Church catholic–in heaven and on earth, in all times and places–in the one Body of Christ, in the unity of the Spirit in the bond of peace. Just as well have all been called to the one hope that belongs to our call, “one Lord, one faith, one baptism, one God and Father of all, who is over all and through all and in all.” And the Feast of All Saints shared with the final Sundays of the Church Year an eschatological focus on the life everlasting and a confession that “the sufferings of this present time are not worth comparing with the glory that is to be revealed to us.” In all of these emphases, the purpose of this feast is to fix our eyes upon Jesus, the author and perfecter of our faith, that we might not grow weary or fainthearted.