"In the divinely inspired scriptures yeast always signifies wickedness and sin. Our Lord Jesus Christ, warning his holy disciples to be on their guard, said: Beware of the yeast of the scribes and Pharisees. And Paul in his great wisdom wrote that those who have once been sacrificed should put far from them the yeast of impurity that corrupts mind and heart. Purify yourselves of the old yeast, he urged, and become a fresh batch of bread, since you really are unleavened.
"This urgent plea prompted by concern for our well-being shows that spiritual communion with Christ the Savior of us all is not only a benefit to us but also a real need. It also shows how important it is for us to keep our minds pure by refraining from sin and washing away every stain. In a word, we must avoid everything that defiled us in the past, for it is then, when no fault of ours bars the way and we are wholly free from reproach, that we shall open the way to this communion with Christ."
"To this sacrifice we must add...." These "synergistic" words alone are anathema for many Christians. But the problem with saying our acts of charity "detract from the finished work of Christ" is that the finished work of Christ consists precisely in us adding our acts of charity to Christ's sacrifice. Our works do not enhance the propitiatory efficacy of Christ's passion, but they do manifest and realize it, as expiation and worship, in the world, before man and God.
Most of us, I think, realize this (put in fairly technical terms of unmerited enabling grace and charity as the fruit of grace, etc.), but I love how Cyril puts the same abstract truth into such vibrant, biblical language. Christ's sacrifice of pure charity unites us to his holiness and life. In this union, our acts of charity not only derive from his love, but also in fact enhance (or deepen or actualize) the power of his love in us. Let me be so bold as to say we have no salvation without working in holy union with the holy sacrifice of Christ -- for salvation consists precisely in this co-operation. There can be no detraction from Christ's work by our acts of charity since exactly this is Christ's work. Our faith in Christ's redemption is not opposed to our obedience to him, since our faith roots itself precisely in the fact that Christ has redeemed us to obey him holily.
To lean on Cyril's vibrant imagery, Christ is the baker and bread is made to smell and taste of the excellence of the baker's finished work. A fine loaf of bread doesn't detract from its Baker's glory simply because it is excellent. Quite the contrary. What detracts most savagely from the Baker’s glory is a loaf that stinks and never fills the house with its aroma. Our sin, not our holiness, is the true detraction from Christ’s finished work.
Cyril of Alexandria (AD 376-444), Paschal Homilies 19, 2: PG 77, 824-825.
Cyril was a monk, priest, bishop and patriarch of Alexandria. He worked at the Council of Ephesus where he fought against Nestorius who taught the heresy that there were two persons in Christ. He was also a major catechetical writer, a leading Greek Father of the Church and a Doctor of the Church. And I like the cut of his jib.