John Gill Commentary Hebrews 9:17

"For a testament is of force where there hath been death: for it doth never avail while he that made it liveth." — Hebrews 9:17 (ASV)

For a testament is of force after men are dead
The necessity of Christ's death is here urged, from the nature and force of a testament or will, among men, which does not take place, and cannot be executed, till a man is dead.

Otherwise it is of no strength at all whilst the testator lives;
no claim can be made by the legatees for the part they have in it, nor can any disposition be made by the executor of it;

not that hereby is suggested, that the testament or will of God was uncertain and precarious till the death of Christ, and subject to change and alteration as men's wills are till they die; nor that the inheritance could not be enjoyed by the Old Testament saints; for it is certain, it was entered upon by them before the death of Christ; but the sense is, that there was a necessity of it, that the saints right to it, upon the foot of justice, might be evident by it.