Ritual Purity in Judaism
Ritual purity in Judaism is one of the oldest and most fundamental concepts. A kohen, or priest, cannot enter the Tabernacle unless he is ritualistically pure. Perhaps the most serious form of ritualistic impurity is acquired through contact with a corpse. It is believed that only the ashes of a sacrificed red heifer can remove such uncleanness.
The red heifer
The existence of a pure, perfect red heifer is an important part of one of the cornerstone beliefs of Judaism. Bible scripture, specifically Numbers 19:2, states that the Lord spoke unto Moses and Aaron and ordered them to find a red heifer "without spot or blemish", and "upon which never came yoke."
Sacrifice and ashes
The Lord further exhorted Moses and Aaron to bring the red heifer to Eleazar the Priest, so that he might sacrifice her outside the camp, and "sprinkle of her blood directly before the tabernacle seven times." Afterward, the heifer was burned, and her ashes gathered up and stored for later use in a so-called "water of separation," which was believed could remove ritualistic uncleanness.
It is believed that no flawless red red heifer (defined as having no more than two hairs of any other color than red) has been born in Israel since the destruction of the Second Temple. Jews believe that the prophesied Third Temple cannot get built until the kohen can once again achieve ritualistic purity.
As Jews believe that the Messiah cannot come until the Third Temple is built, there is a great deal of interest in obtaining a perfect red heifer through selective breeding, or modern reproductive technologies. To date, such efforts have proven disappointing, as every red heifer born in Israel in recent times has not fit all of the Rabbinical criteria for purity.
An interesting question is what happens if a pure red heifer is once again found. Perhaps the best answer is that only God, Himself, knows for sure.