The word "it" refers to the act of God choosing Jesus to be the eternal priest. This can be stated in positive form, and it can be stated clearly who made the oath. Alternate translation: "And God did not choose this new priest without swearing an oath!" or "And God showed how important it was by swearing an oath!" (See: [[rc://en/ta/man/jit/figs-explicit]] and [[rc://en/ta/man/jit/figs-doublenegatives]])