If the divinity of Christ was a doctrine held by the Church right from the start, why was it disputed in the 4th century? Any dogma can be come challenged, even by well-meaning people–it’s why we have an infallible teaching authority to call all back to the unity of faith.
St. Thomas Aquinas went back and forth and ultimately affirmed it–unfortunately, he argued against it in his most famous work, the Summa (although, in his period of denial, he still believed her to be sanctified in the womb immediately after conception).
First, he affirmed it: “Such was the purity of the Blessed Virgin Mary, who was exempt from both original and actual sin.” [Com. in I Sent, d. 44, q. 1, a. 3, ad 3]
Later, in the Summa, he explicitely denies it: “The Blessed Virgin did indeed contract original sin.” [Summa theologiae IIIa, q. 27, a. 2, ad 2]
Ultimately, though, he accepted it: “For she was most pure because she incurred the stain neither of original sin nor of mortal sin nor of venial sin.”[Expositio super salutatione angelica]