By this He exhorts those that come late to the heavenly word, to haste to ascend to perfection before many whom they see to have grown old in the faith. This sense may also overthrow those that boast to have been educated in Christianity by Christian parents, especially if those parents have filled the Episcopal see, or the office of Priests or Deacons in the Church; and hinder them from desponding who have entertained the Christian doctrines more newly.
It has also another meaning; the “first,” are the Israelites, who become last because of their unbelief; and the Gentiles who were “last” become first. He is careful to say, “Many;” for not all who are first shall be last, nor all last first. For before this have many of mankind, who by nature are the last, been made by an angelic life above the Angels; and some Angels who were first have been made last through their sin.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).