The "certain man" is God the Father.
The first son is the Gentiles, harlots, and publicans. This is because, although they did not have the truth at the beginning and were sinners, they accepted the faith when it was given to them, as did the first son, who refused at first and later obeyed. The second son, who said "I go but did not go, is the Jews, who had the promise from the beginning but did not keep it and fell away.
He was showing the Pharisees that the harlots and publicans and sinners would enter into the kingdom of heaven before them, as the sinners accepted St. John and the truth when it was come, while they rejected it and sought only after their own power.
"Thus much prefaced, the Lord brings forward a parable, to convict them of their irreligion, and show them that the kingdom of God should be transferred to the Gentiles." (St Jerome)
The Jews, by their own words pronounced judgement on themselves: "Whether of them twain did the will of his father? They say unto him, The first. Jesus saith unto them, Verily I say unto you, That the publicans and the harlots go into the kingdom of God before you." (Matthew 21:31)
To work in the vineyard is to do righteousness.
"Whence others think that the parable does not relate to Gentiles and Jews, but simply to the righteous and to sinners. These by their evil deeds had rejected God's service, but after received from John the baptism of repentance; while the Pharisees who made a show of righteousness, and boasted that they did the law of God, despising John's baptism, did not follow his precepts." (St. Jerome)
Matthew the Apostle and Evangelist was a former publican.
Mary of Bethany, the sister of Lazarus, was a former harlot, and one who anointed Jesus' feet before his crucifixion.
Macedonia is in Asia Minor.
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