Psalm 5:13: "For thou wilt bless the just. O Lord, thou hast crowned us, as with a shield of thy good will."
Other times it is said of men.
Luke 2:14: "Glory to God in the highest; and on earth peace to men of good will."
And St. Paul shows us that when it is said of men, it is still to be attributed to God (as well as all supernatural good that men may perform).
Philippians 2:13: "For it is God who worketh in you, both to will and to accomplish, according to his good will."
Ephesians 6:7: "With a good will serving, as to the Lord, and not to men."
So what exactly is good will?
Simple. Good will is the desire to love God for God above all other things, including money, fame, parents, brothers, sisters, spouse, children. It is the willingness to obey God even if He should command us as He commanded Abraham to sacrifice his only child to Him.
And it produces the the fruit of faith, a firm resolve to believe all the words of God, knowing that the Spirit of Truth utters not any falsehood.
Ecclesiasticus 2:10: "Ye that fear the Lord, love him, and your hearts shall be enlightened."
Psalms 144:20: "The Lord keepeth all them that love him; but all the wicked he will destroy."
It is a true spirit of sacrifice, cultivated by faith, hope and charity, and it cannot truly begin to manifest itself in any soul which has not first received the fear of the Lord, which stands in awe at both the mercy of God and His great and terrible justice.
Sophonias 2:11: "The Lord shall be terrible upon them, and shall consume all the gods of the earth: and they shall adore him every man from his own place, all the islands of the Gentiles."
Jeremias 2:19: "Thy own wickedness shall reprove thee, and thy apostasy shall rebuke thee. Know thou, and see that it is an evil and a bitter thing for thee, to have left the Lord thy God, and that my fear is not with thee, saith the Lord the God of hosts."
Good will necessarily includes hatred of sin and the desire to never offend God, especially by sins against His true Faith, or by associating with those who commit such sins.
1 Corinthians 5:11: "But now I have written to you, not to keep company, if any man that is named a brother, be a fornicator, or covetous, or a server of idols, or a railer, or a drunkard, or an extortioner: with such a one, not so much as to eat."
2 Thessalonians 3:6: "And we charge you, brethren, in the name of our Lord Jesus Christ, that you withdraw yourselves from every brother walking disorderly, and not according to the tradition which they have received of us."
2 John 1:9-10: "Whosoever revolteth, and continueth not in the doctrine of Christ, hath not God. He that continueth in the doctrine, the same hath both the Father and the Son. If any man come to you, and bring not this doctrine, receive him not into the house nor say to him, God speed you."
Titus 3:10-11: "A man that is a heretic, after the first and second admonition, avoid: Knowing that he, that is such an one, is subverted, and sinneth, being condemned by his own judgment."
It is necessary for the salvation of each human person to learn the true Catholic Faith, and convert to it, outside of which none at all can attain salvation.