This mystery the apostle refers to Christ and the Church. Just as "that which is born of the flesh is flesh, so that which is born of the spirit is spirit" not only in respect of its birth but also of what is acquired by learning. Thus "the children also are holy," they are well-pleasing to God, in that the Lord's words bring the soul as a bride to God. Fornication and marriage are therefore different things, as far apart as God is from the devil.
Source: The Stromata Book 3