For if the blood of bulls and of goats, and the ashes of an heifer sprinkling the unclean, sanctifieth to the purifying of the flesh: How much more shall the blood of Christ, who through the eternal Spirit offered Himself without spot to God, purge your conscience from dead works to serve the living God? – Hebrews 9:13,14

The heifer was not a spiritual, but a carnal offering. The creature knew nothing of what was being done, it was the involuntary victim. But Christ was under the impulses of the Holy Spirit, which were poured upon Him and He was moved by Him to render up Himself a sacrifice for sin. Hence somewhat of the greater efficacy of His death, for the willingness of the Sacrifice greatly enhanced its value. To give you another and probably a better interpretation of the words, there was the Eternal Spirit linked with the manhood of Christ, our Lord, and by Him, He gave Himself unto God. He was God as well as man, and that Eternal Godhead of His lent an infinite value to the sufferings of His human frame, so that He offered Himself as a whole Christ, in the energy of His eternal power and Godhead.

Oh, what a Sacrifice is that on Calvary! It is by the blood of the Man Christ that you are saved, and yet it is written, “The Church of God which He”—that is God— “has redeemed with His own blood.” One who is both God and man has given Himself as a Sacrifice for us! Is not the Sacrifice inconceivably greater in the fact than it is in the type (the red heifer)? Ought it not most effectually to purge our conscience? After they had burnt the heifer, they swept up the ashes. All that could be burnt had been consumed. Our Lord was made a sacrifice for sin. What remains of Him? Not a few ashes, but the whole Christ, which still remains, to die no more, but to abide unchanged forever! He came uninjured through the fires and now He always lives to make intercession for us! It is the application of His eternal merit which makes us clean and is not that eternal merit inconceivably greater than the ashes of an heifer can ever be? ~ C.H. Spurgeon

https://www.blueletterbible.org/comm/spurgeon_charles/sermons/1481.cfm