This week's Parsha, Yitro, describes the greatest day in human history. The revelation at Mount Sinai, where every Jew heard G-d speak, is one of the main proofs of the truth of Judaism.
The first two of the Ten Commandments were heard by all. Many are not aware that the first commandment of "I am the L-rd Your G-d," teaches the obligation to believe in the existence of G-d. In other words, if one is asked what the source in the Torah is that G-d exists, the answer is, "I am Hashem that took you out of Egypt."
The של״ה הקודש says that in addition to this belief we are to realize Hashem's uniqueness. If Hashem exists, we exist. If Hashem does not exist, G-d forbid, we do not exist. Everything is dependent on Hashem. This is what is meant by אין עוד מלבדו, that is meant to be a kind of mantra that we are to constantly repeat. Shabbat Shalom