55 Leah, too, got children by her handmaid, from the desire of having a numerous family. Zilpah, her handmaid, is, interpreted, an open mouth. So Leah's handmaid represents those who are spoken of in Scripture as engaging in the preaching of the gospel with open mouth, but not with open heart. Thus it is written of some: "This people honor me with their lips, but their heart is far from me." To such the apostle says: "Thou that preachest that a man should not steal, do you steal? Thou that sayest a man should not commit adultery, do you commit adultery?" But that even by this arrangement the free wife of Jacob, the type of labor or endurance, might obtain children to be heirs of the kingdom, the Lord says: "What they say, do; but do not after their works." And again, the apostolic life, when enduring imprisonment, says: "Whether Christ is preached in pretence or in truth, I therein do rejoice, yea, and will rejoice." It is the joy of the mother over her numerous family, though born of her handmaid.
Source: Reply to Faustus the Manichaean (New Advent)