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? – Heb 9:13-14

The sprinkling of the ashes of the heifer upon the unclean was not comprehensible as to its effect by anybody who received it. I mean that there was no obvious connection between the cause and the effect. Supposing an Israelite had been unclean and had been sprinkled with this water? He might now go up to the house of the Lord—but would he see any reason for the change? He would say, “I have received the water of separation, and I am clean, but I do not know why the sprinkling of those ashes should make me clean except that God has so appointed.” Brethren, you and I know how it is that God has made us clean, for we know that Christ has suffered in our place! Substitution explains the mystery and, therefore, it has much more effect upon the conscience than an outward, ritualistic form which could not be explained. Conscience is the understanding exercised upon moral subjects and that which convinces the understanding that all is right soon gives peace to the conscience.

As the ashes of the heifer were for all the camp, so are Christ’s merits for all His people. As they were put where they were accessible, so may you always come and partake of the cleansing power of Christ’s precious Atonement. As a mere sprinkling made the unclean clean, even so may you come and be cleansed even though your faith is but little and you seem to get but little of Christ. O Brothers and Sisters, the Lord God in His infinite mercy gives you to know the power of the great Sacrifice to work peace in you—not after three or seven days, but at once! And peace not merely for a time, but forever! ~ C.H. Spurgeon

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