John Chrysostom
Nothing pleases God more than acts of charity. This is the reason priests, rulers, and prophets were consecrated with oil: they received this anointing as a symbol of God's compassion. Additionally, through this act, they were reminded that a ruler should embody greater mercy; it illustrated that the Spirit also comes upon individuals to provide grace, as God shows compassion to humanity. "Thou hast mercy on all," it is proclaimed, "because Thou canst do all things” (Prem. 11:24). Therefore, they were anointed with oil! The priesthood was instituted to embody mercy, and kings were also set apart with this anointing. If one seeks to commend the leader, the highest praise arises from acknowledging their mercy, for mercy is a fundamental trait of true authority.
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Bear in mind that the world was created through grace and strive to emulate the Lord.
When it was time to anoint an individual, the gift of the Spirit was poured out upon the oil, empowering the prophets to carry out these sacred acts.