Why do we fast for forty days?
Formerly many believers approached the sacraments without any particular preparation, especially at the time when Christ first gave them to us.
But when the fathers realized the harm that could result from such neglect, they took counsel together and decreed that a period of forty days of fasting be set aside, during which the people would meet to pray and listen to the word of God.
During this Lenten season each of the faithful would undergo a thorough purification by means of prayer, almsgiving, fasting, watching, repentant tears, confession, and every other remedial measure.
Then when they had done all in their power to cleanse their consciences, they could approach the sacraments.
[…] So, when someone asks you why you fast, you should not answer: because of the Passover, or because of the Cross.
Neither of these is the reason for our fasting. We fast because of our sins, since we are preparing to approach the sacred mysteries.
Moreover, the Christian Passover is a time for neither fasting nor mourning, but for great joy, since the Cross destroyed sin and made expiation for the whole world.
It reconciled ancient enmities and opened the gates of heaven. It made friends of those who had been filled with hatred, restoring them to the citizenship of heaven.
Through the Cross our human nature has been set at the right hand of the throne of God, and we have been granted countless good things besides.
Therefore we must not give way to mourning or sadness; we must rejoice greatly instead over all these blessings.
Listen to the exultant words of Saint Paul: God forbid that I should boast of anything but the Cross of our Lord Jesus Christ. And elsewhere he writes: God shows his own love for us because when we were still sinners Christ died for our sake.
Saint John’s message is the same. God loved the world so much, he declares, and then, passing over every other manifestation of God’s love, he comes at once to the crucifixion.
God loved the world so much that he gave his only Son, that is, he gave him up to be crucified, so that those who believed in him might not perish but might have eternal life.
If, then, the Cross has its foundation in love and is our glory, we must not say we mourn because of the Cross. Far from it. What we have to mourn over is our own sinfulness, and that is why we fast.
John Chrysostom (c.347-407): Oratio 3 Adversus Iudaeos (PG 48, 867-868); from the Monastic Office of Vigils, Ash Wednesday, Year 2.