In the Orthodox Christian ceremony for "A Woman on the Fortieth Day" (Sarandismos), the mother, having been cleansed and washed, stands at the church entrance with her infant. On this fortieth day of life the infant, whether it be a firstborn or not [Ex. 13:2], is brought to the temple to be churched, that is, to make a beginning of being taken into the Church. A prayer is made on behalf of the mother too that her bodily defilement and the stains of her soul be washed away, and that she be made worthy of the Communion of the holy body and blood of Christ.
According to the Law of Moses in the Old Testament, the loss of blood (for women) or seed (for men) required ritual purification since it was looked upon as a diminution of the life principle and involved exclusion from Israel's religious life prior to purification. According to the law: "...the life of flesh is in its blood..." [Lev. 17:11; Deut. 12:23]; thus the uncleanness came from neither conception nor childbirth. It was in delivery that the mother's vitality (linked with her blood) was diminished. Hence, she was "separated" from the Lord, the Source of Life, until her integrity was restored by purification.
The flux, being a natural process instituted by God, and having been permitted to occur thus after the transgression, is neither a sin nor an uncleanness; "for these things are not truly sin nor uncleanness", according to Saint John Chrysostom (+ 407). The Apostolic Constitutions (Bk. IV, Ch. 26) assert that childbirth cannot pollute a woman's nature or separate her from the Holy Spirit; but only impiety and an unlawful act can do so. If actions that occur naturally and without exercise of human will are unclean, how much more unclean are sins, which we do with the exercise of our will? If God has pronounced these fluxes as "unclean", it was done in order to prevent the husband from having sexual relations with the new mother as a means to protect her in this time of weakness and possible embarrassment. This promotes the modesty of men and the honor of women, according to Isidore; and awe of the law of nature, according to Philo. Both the ancients and medical science today know that children conceived during the time of flux are often weaker in nature. So, for all these reasons, reverence and fear were instilled not only into women, but much more into the impetuous vehemence of the natural instinct of men.
More important, these laws reminded the Israelites that sex was not part of their worship, for men could not worship until they cleansed themselves. All this was done so that they might be set apart from the other ancient cultures and their idolatrous neighbors, for whom fertility rites and temple prostitutes formed an important part of worship.
(Holy Apostles Convent, The Great Synaxaristes of the Orthodox Church, pp. 72-73.)