>>2640310 was right: the OT sacrifices merely made worshippers ritually clean so that they could enter the temple courts, but a person is clean in the eyes of God through the death/blood of Christ, which cleanses from the defilement of sin when it is received by faith.
Also "the ashes of a heifer" refer to the particular type of cow used in the temple worship–its ashes were mixed with water, and this water was used for cleansing–in particular when a person had been healed of leprosy, I think, he had to be sprinkled with this water before he could be readmitted to the temple. Since God was present in the temple, what was unclean and unholy could not enter without risking death (like in Raiders of the Lost Ark, where the guy opens up the ark of the covenant and his face melts off.)
The water with the ashes of the immolated heifer were symbolic of Christian baptism, where the simple water is infused with the death of Christ, and so cleanses those who are baptized and believe from sin and uncleanness; but where the ash-water merely cleansed externally and ritually, the one who is baptized and believes is made clean from all sin.