Cassini, St Thomas isn't saying that we stop looking for scientific discoveries; he's only saying that if we can't discover everything, then this isn't an obstacle to Faith.
I'm sure the saints of the Middle Ages could never fathom the modern discoveries of the atom, or DNA, fingerprinting, etc. All of which God allowed to be found. And none of us, St Thomas included, knows what God will allow to be discovered (in the future) or not.