54 Thus Jacob has two free wives; for both are daughters of the remission of sins, or of whitening, that is, of Laban. One is loved, the other is borne. But she that is borne is the most and the soonest fruitful, that she may be loved, if not for herself, at least for her children. For the toil of the righteous is specially fruitful in those whom they beget for the kingdom of God, by preaching the gospel amid many trials and temptations; and they call those their joy and crown for whom they are in labors more abundantly, in stripes above measure, in deaths often, — for whom they have fightings without and fears within. Such births result most easily and plentifully from the word of faith, the preaching of Christ crucified, which speaks also of His human nature as far as it can be easily understood, so as not to hurt the weak eyes of Leah.
Rachel, again, with clear eye, is beside herself to God, and sees in the beginning the Word of God with God, and wishes to bring forth, but cannot; for who shall declare His generation? So the life devoted to contemplation, in order to see with no feeble mental eye things invisible to flesh, but understood by the things that are made, and to discern the ineffable manifestation of the eternal power and divinity of God, seeks leisure from all occupation, and is therefore barren.
In this habit of retirement, where the fire of meditation burns bright, there is a want of sympathy with human weakness, and with the need men have of our help in their calamities. This life also burns with the desire for children (for it wishes to teach what it knows, and not to go with the corruption of envy), and sees its sister-life fully occupied with work and with bringing forth; and it grieves that men run after that virtue which cares for their wants and weaknesses, instead of that which has a divine imperishable lesson to impart.
This is what is meant when it is said, "Rachel envied her sister." Moreover, as the pure intellectual perception of that which is not matter, and so is not the object of the bodliy sense, cannot be expressed in words which spring from the flesh, the doctrine of wisdom prefers to get some lodging for divine truth in the mind by whatever material figures and illustrations occur, rather than to give up teaching these things; and thus Rachel preferred that her husband should have children by her handmaid, rather than that she should be without any children. Bilhah, the name of her handmaid, is said to mean old; and so, even when we speak of the spiritual and unchangeable nature of God, ideas are suggested relating to the old life of the bodily senses.
Source: Reply to Faustus the Manichaean (New Advent)