"And I appeared to Avrohom, Yitzchok and Yaakov with the Name "Kel Shakai" and My Name "Ado-nai" I did not make known to them" (Shemos 6, 3). Rashi notes that Hashem did not say that He "did not mention to them" the name Ado-nai. Rather, Hashem said that He "did not make Himself known" by the name Ado-nai. The Avos never saw the manifestation of the middah connoted by this Name of Hashem. Ado-nai represents the fulfillment of what Hashem has previously guaranteed. Hashem was telling Moshe that He made numerous promises and also made a covenant with Avrahom that He would give his children Eretz Yisroel, but the Avos never saw the fruition of this promise. Now He would reveal Himself with the Name Ado-nai, and fulfill to Moshe and his generation what He promised the Avos.
According to Rashi's explanation, the Torah is letting us know that Hashem has two distinct names: "Kel Shakai" which denotes a promise to be fulfilled in the future, and "Ado-nai" which denotes the fulfillment of a previous promise. Rav Wolbe (Shiurei Chumash) asks why a promise and its fulfillment need two separate Names; are they not in reality one extended process? He explains that a "havtacha" - promise for the future, is not merely a means of reaching its fulfillment; it is a middah by way of which Hashem runs the world. The generations of our Avos were promised great things in the future, and they had to live with total faith that Hashem would ultimately fulfill what He promised. Their avodah was to live their lives in a way that showed their belief and complete reliance on Hashem's word.
In truth, our avodah is very similar to that of the Avos in this respect. We must live our lives with complete faith that all of our actions will be duly rewarded in the next world. We cannot see the reward nor the punishment that awaits those who perform the mitzvos or neglect them. Nevertheless, we must place our belief in Hashem's havtacha that ultimately we will be paid in full for all that we have done. We live in this world with the name of Kel Shakai - the promise of future compensation, and in the next world will we be able to perceive the manifestation of His name "Ado-nai."
The two Names of Hashem describe two distinct ways that He manifests Himself in our world, and we must live our lives accordingly. The more we strengthen our belief in the ultimate reward, the easier it we will be to fulfill our obligations of Torah, avodah and gemilas chassadim.