The definition of blessedness as drawn from the Scriptures is considered and proved. It cannot be enhanced by external good fortune, nor can it be weakened by misfortune.
8. As, then, knowledge, so far as it stands alone, is put aside either as worthless, according to the superfluous discussions of the philosophers, or as but an imperfect idea, let us now note how clearly the divine Scriptures explain a thing about which we see the philosophers held so many involved and perplexing ideas. For the Scriptures state that nothing is good but what is virtuous, and declare that virtue is blessed in every circumstance, and that it is never enhanced by either corporal or other external good fortune, nor is it weakened by adversity. No state is so blessed as that wherein one is free from sin, is filled with innocence, and is fully supplied with the grace of God. For it is written: “Blessed is the man that has not walked in the counsel of the ungodly, and has not stood in the way of sinners, and has not sat in the seat of pestilence, but in the law of the Lord was his delight.” And again: “Blessed are the undefiled in the way, who walk in the law of the Lord.”
9. Innocence, then, and knowledge make a man blessed. We have also noted already that the blessedness of eternal life is the reward for good works. It remains, then, to show that when the patronage of pleasure or the fear of pain is despised (and the first of these one abhors as poor and effeminate, and the other as unmanly and weak), that then a blessed life can rise up in the midst of pain. This can easily be shown when we read: “Blessed are you when men shall revile you and persecute you and shall say all manner of evil against you for righteousness' sake. Rejoice and be exceeding glad, for great is your reward in heaven; for so persecuted they the prophets which were before you.” And again: “He that will come after Me, let him take up his cross and follow Me.”
Source: On the Duties of the Clergy (New Advent)