Charles H. Spurgeon on Psalms 110:3

Thy people. That is, those whom thou dost receive from thy Father, and, by setting up the standard and ensign of the Gospel, gather to thyself. "Shall be willing." The word is willingness, that is, a people of great willingness and devotion, or (as the original word is elsewhere used, Ps 119:108), shall be freewill offerings unto thee. The abstract being put for the concrete, and the plural for the singular, notes how exceeding forward and free they should be; as the Lord, to signify that his people were most rebellious, saith, that they were rebellion itself, Eze 2:8. So then the meaning is, thy people shall, with most ready and forward cheerfulness, devote, consecrate, and render up themselves to thy government as a reasonable sacrifice, shall be of a most liberal, free, noble, and unconstrained spirit in thy service, and shall be voluntaries in the wars of thy kingdom. Edward Reynolds.