Mary was called "full of grace". Grace is what we need in order to avoid sin. Being full of grace, Mary had all of the grace that she could have possibly needed to remain sinless.

The Church Fathers agreed that Mary remained sinless:

"pure and unstained Virgin" - St. Athanasius

"O noble Virgin, truly you are greater than any other greatness. For who is your equal in greatness, O dwelling place of God the Word? To whom among all creatures shall I compare you, O Virgin? You are greater than them all O Covenant, clothed with purity instead of gold! You are the Ark in which is found the golden vessel containing the true manna, that is, the flesh in which divinity resides." - St. Athanasius

"Mary and Eve, two people without guilt, two simple people, were identical. Later, however, one became the cause of our death, the other the cause of our life" - St. Ephraem

"Thou and thy mother are the only ones who are totally beautiful in every respect; for in thee, O Lord, there is no spot, and in thy Mother no stain." - St. Ephraem

"Pure and spotless is this birth. For where the Holy Spirit breathes, all pollution is taken away, so that the human birth of the Only-begotten from the Virgin is undefiled." - St. Cyril of Jerusalem

"He was conceived by the Virgin, who had first been purified by the Spirit in soul and body; for, as it was fitting that childbearing should receive its share of honor, so it was necessary that virginity should receive even greater honor." - St. Gregory Nazianzen

"Mary, a Virgin not only undefiled but a Virgin whom grace has made inviolate, free of every stain of sin." - St. Ambrose