Theology of St. Thomas - Necessity of Grace

Our last end, St. Thomas says, is the beatific vision.

For as the ultimate beatitude of man consists in the use of his highest function, which is the operation of his intellect; if we suppose that the created intellect could never see God, it would either never attain to beatitude, or its beatitude would consist in something else beside God; which is opposed to faith. For the ultimate perfection of the rational creature is to be found in that which is the principle of its being; since a thing is perfect so far as it attains to its principle. Further the same opinion is also against reason. For there resides in every man a natural desire to know the cause of any effect which he sees; and thence arises wonder in men. But if the intellect of the rational creature could not reach so far as to the first cause of things, the natural desire would remain void.

Hence it must be absolutely granted that the blessed see the essence of God (ST I Q12 A1).

But this is not something that we are able to accomplish on our own strength. I'll spare you the lengthy quotation from ST I Q12 A4, but the point there is that we are unable to do so because we are mere creatures, and God is infinitely above us. Consequently if we are going to see God, as he has already argued, we are only going to be able to do so by the grace of God. But if this is true even of the holy angels, as St. Thomas insists (ST I Q62 A2), then obviously it is especially true for us fallen men.

Aside from recognizing that we are completely dependent upon God for our salvation, and ought to live our lives in ways that reflect this dependency, it's worth pointing out that St. Thomas is affirming that our salvation comes to us sola gratia - by grace alone. Of course, as we have already seen, this is exactly what Trent affirmed (Protestant denials notwithstanding).