Protestant anti-Catholic polemicist Eric Svendsen, who fancies himself an expert on Catholic Mariology, wrote in his doctoral dissertation from 2000:

Post-Tridentine Catholic theology “went so far as to develop arguments from ‘fittingness,’ ... This ‘fittingness’ argument is usually traced to Duns Scotus [c. 1265-1308]. Excesses such as this lent credibility to the Protestant charge that Catholics ‘divinized’ Mary in an idolatrous way.”

Svendsen was probably referring to a piece by Duns Scotus (1266-1308) entitled, On the Fittingness of the Immaculate Conception. But Duns Scotus in his first paragraph of this treatise, cited St. Anselm (c. 1033-1109): “It was fitting that the Virgin should be resplendent with a purity greater than which none under God can be conceived.” In any event, it was actually the writers of the New Testament who thought in these terms:

• Hebrews 2:10 (RSV) For it was fitting that he, for whom and by whom all things exist, in bringing many sons to glory, should make the pioneer of their salvation perfect through suffering.
• Hebrews 7:26 For it was fitting that we should have such a high priest, holy, blameless, unstained, separated from sinners, exalted above the heavens.
• Matthew 3:15 But Jesus answered him, “Let it be so now; for thus it is fitting for us to fulfill all righteousness.”
• 1 Corinthians 11:13 Judge for yourselves; is it proper for a woman to pray to God with her head uncovered?
• Ephesians 5:3-4 But fornication and all impurity or covetousness must not even be named among you, as is fitting among saints. Let there be no filthiness, nor silly talk, nor levity, which are not fitting; but instead, let there be thanksgiving.
• Colossians 3:18 Wives, be subject to your husbands, as is fitting in the Lord.
• 2 Thessalonians 1:3 We are bound to give thanks to God always for you, brethren, as is fitting …
• 1 Timothy 2:6 who gave himself as a ransom for all, the testimony to which was borne at the proper time.
• 1 Timothy 6:15 and this will be made manifest at the proper time by the blessed and only Sovereign, the King of kings and Lord of lords,
• Titus 1:3 and at the proper time manifested in his word through the preaching with which I have been entrusted by command of God our Savior …

Dictionary.com defines “fitting” as “suitable or appropriate; proper or becoming.” Thesaurus.com provides 21 synonyms for “fitting” including “apt,” “proper,” “correct,” “desirable” and “seemly.” We see that supposedly exclusively Catholic “fittingness” is a frequent and explicit biblical teaching, and that Catholic thinking is thoroughly, comprehensively and deeply biblical. Hence, Blessed Pope Pius IX, in declaring the dogma of the Immaculate Conception of Mary in 1854 (Ineffabilis Deus), expressed these explicitly biblical terms (four times):

And indeed it was wholly fitting that so wonderful a mother should be ever resplendent with the glory of most sublime holiness and so completely free from all taint of original sin that she would triumph utterly over the ancient serpent.

And hence they affirmed that the Blessed Virgin was, through grace, entirely free from every stain of sin, and from all corruption of body, soul and mind; that she was always united with God and joined to him by an eternal covenant; that she was never in darkness but always in light; and that, therefore, she was entirely a fit habitation for Christ, not because of the state of her body, but because of her original grace.

For it was certainly not fitting that this vessel of election should be wounded by the common injuries, since she, differing so much from the others, had only nature in common with them, not sin. In fact, it was quite fitting that, as the Only-Begotten has a Father in heaven, whom the Seraphim extol as thrice holy, so he should have a Mother on earth who would never be without the splendor of holiness.

After all, an angel declaring that the Blessed Virgin Mary was “full of grace” meant that she was without sin, as a straightforward deduction from the biblical data regarding grace and sin. This is why it is indeed “fitting” that Mary was immaculately conceived, so that she could be freed — entirely by God’s grace, since she had no will or self-consciousness a moment after her conception — from original as well as actual sin.

Likewise, in Munificentissimus Deus, Venerable Pope Pius XII’s declaration of the dogma of Mary’s Bodily Assumption in 1950, we find seven examples of the concept, including citing St. John Damascene (c. 645-c. 749) using the term five times:

Thus St. John Damascene, an outstanding herald of this traditional truth, spoke out with powerful eloquence when he compared the bodily Assumption of the loving Mother of God with her other prerogatives and privileges.

‘It was fitting that she, who had kept her virginity intact in childbirth, should keep her own body free from all corruption even after death. It was fitting that she, who had carried the Creator as a child at her breast, should dwell in the divine tabernacles. It was fitting that the spouse, whom the Father had taken to himself, should live in the divine mansions. It was fitting that she, who had seen her Son upon the cross and who had thereby received into her heart the sword of sorrow which she had escaped in the act of giving birth to him, should look upon him as he sits with the Father. It was fitting that God’s Mother should possess what belongs to her Son, and that she should be honored by every creature as the Mother and as the handmaid of God.’ [Encomium in Dormitionem Dei Genetricis Semperque Virginis Mariae, Hom. II]

The word “fitting” also appears in sections 33 and 36.

So yes, it’s true: “It was fitting that the Virgin should be resplendent with a purity greater than which none under God can be conceived.”