The priest says this prayer quietly before the Gospel at the Divine Liturgy. Many Orthodox Christians also say it before reading the Bible at home.
Illumine our hearts, O God who lovest mankind, with the pure light of thy divine knowledge, and open the eyes of our understanding to the comprehension of the proclamation of thy Gospel. Implant in us, likewise, the fear of thy blessed commandments; that, trampling down all carnal desires, we may pursue a godly life, both thinking and performing such things as are well-pleasing unto thee. For thou art the light of our souls, and of our bodies, O Christ our God, and unto thee do we ascribe glory, together with thy Father who is from everlasting, and thy holy, and blessed, and life-giving Spirit, now, and ever, and unto ages of ages. Amen.
From the Divine Liturgy of St John Chrysostom, in Isabel F. Hapgood's translation (Service Book of the Holy Orthodox-Catholic Apostolic (Greco-Russian) Church, Boston: Houghton, Mifflin, 1906, pp. 88–89; public domain). See the scan