“I said to you a little ago,” he answered, “that you were cunning and obstinate in asking explanations of the parables; but since you are so persistent, I shall unfold to you the meaning of the similitudes of the field, and of all the others that follow, that you may make them known to every one. Hear now,” he said, “and understand them. The field is this world; and the Lord of the field is He who created, and perfected, and strengthened all things; [and the son is the Holy Spirit;] and the slave is the Son of God; and the vines are this people, whom He Himself planted; and the stakes are the holy angels of the Lord, who keep His people together; and the weeds that were plucked out of the vineyard are the iniquities of God's servants; and the dishes which He sent Him from His table are the commandments which He gave His people through His Son; and the friends and fellow-councillors are the holy angels who were first created; and the Master's absence from home is the time that remains until His appearing.”
I said to him, “Sir, all these are great, and marvellous, and glorious things. Could I, therefore,” I continued, “understand them? No, nor could any other man, even if exceedingly wise. Moreover,” I added, “explain to me what I am about to ask you.” “Say what you wish,” he replied. “Why, sir,” I asked, “is the Son of God in the parable in the form of a slave?”
Source: The Pastor (or "The Shepherd") (New Advent)