Because I have been practicing with the choir in preparation for Pascha, I had an opportunity to sing the hymn we sing at midnight:
Thy resurrection oh Christ our savior,
the angels in heaven sing,
enables us on earth
to glorify thee in purity of heart.
For some reason singing that caused me to think about the implications of what it says. It seems obvious and maybe intuitive, but I ended up spending several hours pouring through theology books in an attempt to find a good articulation of the reality.
I ended up with two passages from different theology books that, while perhaps not explicit about purity of heart, lead one on the path to understanding. The first passage relates in detail what happened at the death and resurrection of our Lord. This is taken from Orthodox Dogmatic Theology by Protopresbyter Michael Pomazansky:
Christ, after His death on the Cross, descended in His soul and in His Divinity into hell, at the same time that His body remained in the grave. He preached salvation to the captives of hell and brought up from there all the Old Testament righteous ones into the bright mansions of the Kingdom of Heaven. Concerning this raising up of the righteous ones from hell, we read in the Epistle of St. Peter: “For Christ also hath once suffered for sins, the just for the unjust, that He might bring us to God, being put to death in the flesh, but quickened by the Spirit; by which also He went and preached unto the spirits in prison” (1 Peter 3:18-19). And in the same place we read further: “For this cause was the Gospel preached also to them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit” (1 Peter 4:6). St. Paul speaks of the same thing: quoting the verse of the Psalm, “When He ascended up on high, He led captivity captive, and gave gifts unto men,” the Apostle continues: “Now that He ascended, what is it but that He also descended first into the lower parts of the earth? He that descended is the same also that ascended up far above all the heavens, that He might fill all things” (Eph. 4:8-10).
To use the words of St. John Chrysostom, “Hell was taken captive by the Lord Who descended into it. It was laid waste, it was mocked, it was put to death, it was overthrown, it was bound” (Homily on Pascha).
I suppose one might wonder how one could have any purity of heart if one was held captive in hell. That’s somewhat crude, but it’s clear that death and hell are a factor that would prevent us from glorifying Christ in purity of heart.
The second passage I found helpful is from The Mystical Theology of the Eastern Church by Vladimir Lossky:
The way of deification, which was planned for the first man, will be impossible until human nature triumphs over sin and death. The way to union will henceforth be presented to fallen humanity as salvation. This negative term stands for the removal of an obstacle: one is saved from something—from death, and from sin—its root. The divine plan was not fulfilled by Adam; instead of the straight line of ascent towards God, the will of the first man followed a path contrary to nature, and ending in death. God alone can endow men with the possibility of deification, by liberating him at one and the same time from death and from captivity to sin. What man ought to have attained by raising himself up to God, God achieved by descending to man. That is why the triple barrier which separates us from God—death, sin, nature—impassable for men, is broken through by God in the inverse order, beginning with the union of the separated natures, and ending with victory over death. Nicholas Cabasilas, a Byzantine theologian of the fourteenth century, said on this subject: “The Lord allowed men, separated from God by the triple barrier of nature, sin and death, to be fully possessed of Him and to be directly united to Him by the fact he has set aside each barrier in turn: that of nature by His incarnation, of sin by His death, and of death by His resurrection.” This is the reason why St. Paul writes: “The last enemy that shall be destroyed is death” (I Cor 15:26).
This makes it clear that the last barrier is death and thus the resurrection is that event which finally unravels or removes the the last barrier to deification, and thus purity of heart.