There are many come to the faith, yet but few arrive at the heavenly kingdom; many follow God in words, but ignore Him in their daily lives. Immediately two things come to mind. The first, that none should presume concerning himself; for though he be called to the faith, he knows not whether he shall be chosen to the kingdom. Secondly, that none should despair of his neighbor, even though he see him lying in vices; because he knows not the riches of God's mercy.
The morning is our childhood; the third hour may be understood as our youth, the sun as it were mounting to his height is the advance of the heat of age; the sixth hour is manhood, when the sun is steady in his meridian height, representing as it were the maturity of strength; by the ninth is understood old age, in which the sun descends from his vertical height, as our age falls away from the fervor of youth; the eleventh hour is that age which is called decrepit, and doting.
They then who have neglected till extreme old age to live to God, have stood idle to the eleventh hour, yet even these the master of the household calls, and oftentimes gives them their reward before other, inasmuch as they depart out of the body into the kingdom before those that seemed to be called in their childhood.
When He says, The first shall be last, and the last first, He alludes secretly to such as were at the first eminent, and afterwards showed no virtue; and to others who have been reclaimed from wickedness, and have surpassed many. So that this parable was made to quicken the zeal of those who are converted in extreme old age, that they should not suppose that they shall have less than others.
Homiliae in Evangelia #19 (St. Gregory the Great)
Homily 64 on Matthew (St. John Chrysostom)