Until the Exodus from Egypt, the patriarchs had an individual relationship with God, bringing sacrifices wherever and whenever they felt the calling. But when God brought the Children of Israel out of Egypt, what had been a clan, a loose conglomeration of individuals, was transformed into a nation consecrated to serving God. In Egypt, the firstborns were God’s special purview as they were designated to be included in the plague that wiped out all the firstborns in Egypt. By sparing the firstborn of Israel, God set them aside to serve Him. They forfeited this position when they took part in the sin of the Golden Calf. The Tribe of Levi did not take part in the sin and therefore took their place. As a Levi and the leader of the people alongside Moses, Aaron became the archetypal patriarch of all future Kohanim (priests).
Though this may seem a great honor with benefits that included certain tithes and gifts to the Temple, the position came with drawbacks. Unlike their brethren, the Tribe of Levi did not receive a portion of the Land of Israel.
Of course, serving in the Temple brought with it honor, visibly manifested in the glory of the service. But this honor also came with additional restrictions, described in Leviticus (chapter 21). The word kadosh (holy) literally means ‘separate’, so holiness is maintained by separating from anything that may bear a spiritual blemish.
The most notable restrictions are in marriage. The Jewish nation maintains its role as a priestly nation (Exodus 19:6) through clear ancestry and sanctity in marriage and relations. Since the Temple laws required meticulous concern for tradition, God mandated additional restrictions for the Kohanim in marriage, as the stability of a hereditary priesthood was indispensable.
These restrictions seem to be uncomfortably un-Western, reflecting a caste system that runs counter to liberal values of equality and even the American dictum of “All men are created equal.” But it is through these restrictions that Jews in general, and the priests in particular, have maintained their ability to preserve a relationship with God throughout the 2,000-year exile.
By maintaining the generational purity, Kohanim were part of a triumvirate that crowned Israel. The king provided national stability, the judges provided social stability, and the Kohen provided religious stability and continuity
Leviticus 21:7 lists three types of women prohibited to a Kohen: a zonah (Harlot), a chalalah (a woman with a defective Kohen status), and a divorcee. In addition, a Kohen is also forbidden from marrying a convert and a chalutzah (woman designated for levirate marriage). A Kohen who enters into such a marriage loses the entitlements of his priestly status while in that marriage. The Kohen is not permitted to forgo his status and marry a woman prohibited to him.
Modern-day Kohanim are also prohibited from marrying a divorcee (even their own wives which they divorced); a woman who has committed adultery, had been involved in incest, or had relations with a non-Jew; a convert; or the child of two converts. A born-Jewish woman who has had premarital relations may marry a Kohen only if all of her partners were Jewish.
The Talmud rules that if a Kohen married in disregard of the above prohibitions, his marriage would be effective. However, the children are termed chalalim (“disqualified”) and do not possess Kohen status. Also, the Kohen who does so may no longer act as a Kohen or receive any of the benefits and honors befitting a Kohen. For example, he does not get called to the Torah first, nor may he administer the Priestly Blessing. The priesthood itself, however, remains with him and can never be renounced. For this reason, the prohibitions that apply to a Kohen (that he may not enter a cemetery, etc.) still apply to him.
The High Priest, however, may only marry a virgin.
The Shulchan Aruch (code of Jewish law) rules that the daughter of a Jewish mother and non-Jewish father, while considered Jewish, is prohibited from marrying a Kohen. If such a marriage is performed the couple would not have to get divorced, but there is a doubt as to whether their sons would be allowed to serve in the 3rd Temple.