Link.Evangelicals often use the fact that Aquinas (unlike others of his age) did not believe that Mary was entirely sanctified from the moment of her conception to imply that she committed actual, personal sin--as Protestants assert. However, nothing could be further from the truth. Even though Aquinas did not claim that Mary was sanctified from the moment of her conception, he did claim that she was sanctified before her birth, and so never committed personal sin (for unborn children commit no personal sin; cf. Rom. 9:11). --Link.
There is more here: http://www.cin.org/users/james/questions/q052.htm
I should also mention that Thomas Aquinas was a theologian in the Church. In The Catholic Way by Bishop Donald Wuerl he says, "Theologians and scholars teach the word and help the Church to penetrate its full meaning. They are not official teachers in the way that bishops, the successors of the apostles, are; theologians do not receive with the bishops that "sure gift of truth" (Dei Verbum 8) that apostolic witnesses to faith receive. But they are important companions of faith, for bishops look to scholars for appropriate assistance in understanding divine revelation."
Another important point to remember is that in Aquinas' time, the Immaculate Conception was a tradition of the Church but was not a required belief. It did not become doctrine (thus a required belief) until the 19th century.
Here's more: http://www.cin.org/jp960612.html
"Down the centuries, the conviction that Mary was preserved from every stain of sin from her conception, so that she is to be called all holy, gradually gained ground in the liturgy and theology. At the start of the 19th century, this development led to a petition drive for a dogmatic definition of the privilege of the Immaculate Conception."
Im not using Aquinas in this sense. It is simply to refute the notion that the IC is part of the apostolic or ancient church tradition.
Conviction does not equal truth. Islamists and Mormons hold religiously to certain convictions of their theology.