What does the Bible mean by hold fast?

As St Paul says―prove all things―hold fast that which is good. In other words, you are not like a wandering atom in space with no guidance, no power of grasping and understanding truth, but you have been put here with all the powers of the Universe touching you and manifesting through you.

You have been created with a perfect receiving apparatus by which you can, not only receive the great radio-dispensing of Divine wisdom and truth, but can separate it and judge it. For you have had written for you in an unmistakable way, the great book of Nature. You are told from the beginning that only those things you see worked out in Nature are, and can be truths, and that you are the microcosm of the macrocosm, and hence must follow the same laws. And if you do, you will find the same satisfactory results. Therefore, nothing is true to you―be it ever so true to others―unless you can prove it for yourself―unless you can see its corresponding accomplishment worked out for you in the cosmos, and can prove it from the laws of Nature or your own experience.

Harriette Augusta Curtiss and F. Homer Curtiss, Letters from the Teacher, Volume II