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Generally, the writings of Saint Thomas Aquinas are considered by the Catholic Church to be second only to those writings contained in the canon of Sacred Scripture. Given this, much attention ought to be paid to what he has written on every point of theology. His theological writing has guided the Church in the development of doctrine since the middle ages.
Where the Magisterium departs from Aquinas, we ought to assent to what the Magisterium says, as the Church, and not Aquinas, is guaranteed to be protected from error by the Holy Spirit, according to the words of Christ. However, we should try to understand why the Magisterium has departed from Aquinas whenever she does, as such an exercise can only assist us in growing in wisdom and understanding of the faith.
On matters for which the Magisterium has not made official pronouncements, it is safe to believe the same thing any Doctor of the Church has said, whether they are Aquinas or another Doctor. But, you are under no obligation to believe the same things as them unless the Church has said you must. Although you are under no obligation, if you will disagree with them, you ought to be able to explain why.