Charles H. Spurgeon on Psalms 110:4

Thou art a priest. This word, "Thou art," is "verbum constitutivum", a "constituting word", whereon the priesthood of Christ was founded. And it may be considered,

1. As declarative of God's eternal decree, with the covenant between the Father and the Son, whereby he was designed unto this office.

2. As demonstrative of his mission, or his actual sending to the discharge of his office. These words are the symbol and solemn sign of God's conferring that honour upon him, which gave him his instalment.

3. As predictive, for there is included in them a supposition that God would prepare a body for him, wherein he might exercise his priesthood, and which he might offer up unto him. John Owen.