No one can truly say he knows God until he has handled something.

What does it mean? What do I need to handle to know God?

The beauty of the teachings of the Lord is that they are true and that you can confirm them for yourself.

It would be beautiful if you could, but alas, either the teachings are not testable, or the test fails, or there is a simpler explanation.

If we understood the process of creation there would be no mystery about it, it would be all reasonable and plain, for there is no mystery except to the ignorant.

This is true as far as it goes, but note that astrophysicists admire the night sky at least as much as lay folks, despite being able to describe in some detail how the stars shine and galaxies form. So "reasonable" doesn't mean "plain".

This we know by what we have learned naturally since we have had a being on the earth.

I don't understand what this sentence means.

Their not being able to prevail against it does not prove it to be the Kingdom of God, for there are many theories and systems on the earth, incontrovertible by the wisdom of the world, which are nevertheless false.

Does this say that one cannot tell the difference between many models giving the same predictions? Then yes, it is pretty reasonable.