Excerpted from the Handbook for Church Servers of Sergei Bulgakov:
This day the Holy Church commemorates the victory of Orthodoxy over heresies and is why this day is also called the "Sunday of Orthodoxy". The Synaxarion explains that the day celebrates "the restoration of the holy and venerable icons", which happened in first half of the 9th century (in 842) "by the Emperor Michael (the Byzantine Emperor), the holy and blessed Empress Theodora and the Holy Methodius, Patriarch of Constantinople". This commemoration was established to celebrate the final victory of the Holy Church over the iconoclastic heresy. In the hymns for this day the Holy Church, glorifying the holy icons, and also its iconodule adherents and inspiring in us the obligation of venerating icons, sings: "A feast of joy and gladness is revealed to us today. For the teachings of the true Faith shines in all their glory, and the Church of Christ is bright with splendor, adorned with the holy icons which now have been restored; and God has granted to the faithful unity of mind." "Now a pious brightness stretches over all, dispersing the flattery of the impious like a cloud, enlightening the hearts of the pious: come let us fall down with pious wisdom before the honorable icons of Christ worshipping in the Orthodox way", "and with due honor let us venerate the holy icons of Christ, of the all pure Virgin and the saints, whether depicted on walls, on wooden panels or on holy vessels, rejecting the impious teaching of the heretics".
But in this way the Holy Church at the time of victory over the iconoclasts in the struggle against various heresies finally explained and defined in the canons of the Seventh Ecumenical Council the Orthodox Christian teaching that is now a celebrated event and is not called the celebration of iconolatry but the Triumph of Orthodoxy. It does not mean that after the 10th Century heresies would or could not appear any more; but it means that all following heresies, even though they were numerous and various, find their accusation and refutation in the definitions of the Seven Ecumenical Councils.
The victory of Orthodoxy in the Greek Church itself was first celebrated on the first Sunday of Great Lent, and thus the basis of the present Triumph of Orthodoxy is historical. Together with these things the Holy Church through the celebration of the present day means to grant great comfort to those who attend. This intention agrees with the rules of faith and piety and the ascetic effort of the fast. It sees in this action the proof of their living communion according to their faith and life, and finds in this the foundation for prayerful petition to God for them.
From the services of the Lenten Triodion for this day:
Thou who art uncircumscribed, O Master, in Thy divine nature, wast pleased in the last times to take flesh and be circumscribed; and in assuming flesh, Thou hast also taken on Thyself all its distinctive properties. Therefore, we depict the likeness of Thine outward forms, venerating it with an honour that is relative. So we are exalted to the love of Thee, and following the holy traditions handed down by the apostles, from Thine ikon we receive the grace of healing.
As a precious adornment the Church of Christ has received the venerable and holy ikons of the Saviour Christ, of God’s Mother and of all the saints. Celebrating now their triumphant restoration, she is made bright with grace and splendour, and drives away all heretics. With great rejoicing she gives glory unto God who loves mankind, and who for her sake has endured His voluntary Passion.
Leaping up with joy, let us and all the faithful cry aloud today: How marvelous are Thy works, O Christ! How great is Thy might! For Thou hast made us of one mind and brought about our agreement.
Seeing this great blessing that we have received, how the divided members of Christ have been brought to unity, let us clap our hands for joy and praise God who has bestowed peace upon us.
Let us now set out with joy upon the second week of the Fast; and like Elijah the Tishbite let us fashion for ourselves from day to day, O brethren, a fiery chariot from the four great virtues; let us exalt our minds through freedom from the passions; let us arm our flesh with purity and our hands with acts of compassion; let us make our feet beautiful with the preaching of the Gospel; and let us put the enemy to flight and gain the victory.