In a word, we call it loyalty or trust. The essential relationship that exists in the life of a Jew is that which was established at Mount Sinai between us and our Creator. Although every prophet had to initially establish his credentials by performing signs and wonders, these miracles, even those performed by Moses in Egypt, are not the source of our belief. The fault with basing belief on miracles is that while they may be impressive, there often remains some nagging doubt as to whether the miracle was real or merely the result of a fortunate set of natural circumstances. The secular world maintains, for example, that some sort of planetary alignment caused the waters of the Red Sea to split or that the plague of darkness was really a solar eclipse.
Miracles leave room for doubt.
The Rambam writes that when establishing the relationship upon which all of Torah will be built there could be no room for any doubt whatsoever. Therefore, the entire Jewish nation had to hear Hashem speak directly to them, and in order to establish the authenticity of the prophecy of Moses, they had to hear Hashem speak to him as well. A clear allusion to the dangers of doubt can be found in our prayers, as well. When doubts exist there can be no real relationship. It seems that the authors of our Amidah, the silent payer that is the essential centerpiece of our prayer service, could have said the same thing about any of the blessings that we request from HaShem, for example, give us knowledge and remove from us pain and sighing, give us health and wealth and remove pain and sighing, etc.