On the Feast of All Saints, Catholics celebrate the union of the blessed in Heaven, the faithful on earth and the Holy Souls in Purgatory. Protestants, however, deny the existence of the Holy Souls in Purgatory, the intercession of the saints in Heaven and the struggle for salvation fought by the Church Militant on earth.
The unity of the Communion of Saints is scriptural. In 1 Corinthians 12:27, St. Paul said, "[Y]ou are the body of Christ, member for member." In Colossians 1:18, St. Paul affirmed, "[H]e is the head of his body, the Church." The saints are called the Bride of Christ in Ephesians 5:28–30 which reads, "He who loves his own wife, loves himself. ... as Christ also does the Church, because we are members of His body."
The spouse of Christ is composed of members found in three stages of perfection: the Church Triumphant in Heaven, the Church Militant on earth and the Church Suffering in Purgatory.
Saints in Heaven are crowned for their triumph over sin while on earth. They reflect the grace of God in Heaven to the degree which they grew in virtue on earth. Saints on earth are called "militant," as they're still fighting to love God and neighbor while resisting sinful temptations from the world, the flesh and the devil. Saints in Purgatory or poor souls are suffering as they're being purified of the guilt accrued by their venial sins, and as they willingly atone for whatever temporal punishment remains from all the sins, which they committed during their earthly lives.
The three aspects of the universal Church are also in communion with one another because they profess the same faith, obey the same authority and assist one another with their prayers and good works. The Church Militant honors the members of the Church Triumphant, strives to imitate their virtue and benefits from their prayers. The Church Militant also assists the Holy Souls in Purgatory with prayers, fasting, good works, alms and indulgences. The Poor Souls, in return, can benefit from the intercession of the saints in Heaven and pray for the living on earth.
Protestants, with their understanding of Soli Deo Gloria, don't believe that the saints in Heaven are given any special honor for their triumph over sin. They believe in Sola Gratia, so they hold baptized Christians are in a state of total depravity and just dung heaps covered in snow. They also, therefore, hold that saints in Heaven don't reflect God in the degree in which they grew in virtue while on earth.
Protestants believe in Solus Christus, so they deny the intercession of the Saints. They don't believe the Saints in Heaven are aiding the Church Militant on earth with their prayers. As they deny the existence of Purgatory, neither do they believe the members of the faithful on earth aid the poor souls in the Church Suffering or the poor souls in return aid the members of the Church Militant.
Watch the panel discuss the loving unity of the threefold Church in The Download—Communion of Saints.