ALLAH swt says in Quran;
Al-Quran 3:042
“And (remember) when the angels said to Mary, “Oh Mary! Indeed Allah picked and purified you, and preferred you over all the women of the world.””
Surely, it is the mercy and blessing of ALLAH swt on maraym a.s that She was picked to be mother of Esa a.s.
Before her birth, maryam a.s. mother used to pray during her pregnancy duration that whatever she carries in her womb, she will give the child in ALLAH’s way. When Maraym a.s. was born, her mother was a little disappointed to see a girl because she thought that girls are weak, so was her daughter, but ALLAH swt heard her mother’s prayers and made maryam a.s. the only women in the world who was virgin, pure, pious and a mother of prophet of ALLAH (Esa.a.s). this is written in Quran in the following verses;
Al-Quran 3:35-37
“Behold! a woman of 'Imran said: "O my Lord! I do dedicate unto Thee what is in my womb for Thy special service: So accept this of me: For Thou hearest and knowest all things.” When she was delivered, she said: "O my Lord! Behold! I am delivered of a female child!"- and God knew best what she brought forth- "And no wise is the male Like the female. I have named her Mary, and I commend her and her offspring to Thy protection from the Evil One, the Rejected." Right graciously did her Lord accept her: He made her grow in purity and beauty: To the care of Zakariya was she assigned. Every time that he entered (Her) chamber to see her, He found her supplied with sustenance. He said: "O Mary! Whence (comes) this to you?" She said: "From God: for God Provides sustenance to whom He pleases without measure."”
Hazrat Maryam a.s was a very pious lady from her early life. She was given under guardianship of zakaria a.s and she was orderd by ALLAH swt to be pious and to supplicate Him.
Al-Quran 3:044
These are the tales of the unseen (distant past) which We reveal to you. You were not among them, as they cast (lots with) their pens (to determine) who would (be the legal guardian and) assume the care of Mary. You were not among them when they (disagreed and) quarreled
AL-Quran 3:043
“Oh Mary! (With devotion and humility), submit to your Lord. Prostrate and bow down (before Him) along with those who bow down.”
ALLAH swt chose Maraym a.s. and gave her te glad tidings of a son who will be a rooh pure and a Prophet of ALLAH.
Al-Quran 3:045
And (remember) when the angels said, “Oh Mary, Allah gives you the good news of ‘the word’ from Him. His name shall be Messiah, Jesus son of Mary, held in high honor in this world and the hereafter; and he will be one of those nearest (to Allah).”
When Maryam a.s gave birth to hazrat Esa a.s., people of her city accused her of having done sin, but ALLAH swt gave the new born Esa a.s. the power to speak and to tell people that HE was a Prophet of ALLAH swt and son of Maryam a.s. who was a virgin and who was never touched by any man.
Since Maryam a.s. was a pious lady, so ALLAH swt has mentioned in Quran that those who said bad words about Mayram a.s., ALLAH swt punished them.
AL-Quran 4:156
(We punished them) because of their rejection (of the messenger Jesus), and their atrocious utterances of slander against Mary!
(There are still some Christian sects who talk ill about her). It is common belief in christainity that Esa a.s is son of God (na’dubillah), and they say it openly, but Quran condemns it, ALLAH swt warns the christians for what they say in wrong.
Al-Quran 4:171
Oh People of the book! Do not (overstate and) exaggerate your religious beliefs; do not say anything except the truth about Allah. The Messiah, Jesus son of Mary was in fact (nothing more than) the messenger of Allah and His word which He cast upon Mary. (Jesus was) a soul created by Him. So, believe in Allah and His messenger. Do not say (that Allah is) three! Desist! It is better for you (if you desist). Verily, Allah is (only) One. He is (too glorified and) too exalted to have a son. Everything in the heavens and on earth belongs to Him. Allah suffices as a Disposer of all matters and things! (He needs no mediator).
Hazart Maryam a.s. was a very pious lady with clear character, this is the reason she was given the blessing of having a son being pure and pious.