What is the basis for the Catholic teaching that Mary is holy? What is the basis for the Catholic teaching that Mary is holy?  Most other denominations don't hold to doctrines like perpetual virginity. They see Mary as special because she was chosen by God but are unlike Catholicism which seems to teach that she is holy in a way that other denominations don't. I don't understand the basis for this. Can you please explain?
From the Gospel of St. Luke:

41 And it came to pass, that, when Elisabeth heard the salutation of Mary, the babe leaped in her womb; and Elisabeth was filled with the Holy Ghost:
42 And she spake out with a loud voice, and said, Blessed [art] thou among women, and blessed [is] the fruit of thy womb.
43 And whence [is] this to me, that the mother of my Lord should come to me?
44 For, lo, as soon as the voice of thy salutation sounded in mine ears, the babe leaped in my womb for joy.
45 And blessed [is] she that believed: for there shall be a performance of those things which were told her from the Lord.

Note that St. Elisabeth is first filled with the Holy Spirit, then speaks the following lines with authority derived from that Inspiration. And she calls Our Lady “blessed,” which in this context means “Saint.”
In the next versicle, the Magnificat starts (46–55), but one part in verse 48 stands out for our purposes:

48 For he hath regarded the low estate of his handmaiden: for, behold, from henceforth all generations shall call me blessed.

Catholics and Orthodox (and Miaphysites and the Church of the East, as well) all hold that this Evangelic command to “call [Our Lady] blessed” is fulfilled by our counting her as the first among the blessed, and the holiest of all creatures after the Triune God. After all, she was granted that unique privilege to bear the Word of God himself.