Judaism is a system of mitzvot - actions we are to perform or to avoid day in and day out. These mitzvot, based on the Torah and the interpretations of the Sages, reflect Judaism's understanding of what it means to live a Godly life. It is no secret that for most Jews today this system is no longer operative. The reasons for this are many: the secularization of life, the lack of belief in a commanding God, a general disdain for religion and an inability to see any meaning or value in these mitzvot.
I would also venture to say most people, even most "religious" people, do not agree with the point of view expressed by Yeshiyahu Leibovitz and others that one must simply do the mitzvot because we have been so commanded, and not because they have any particular meaning or benefit. Such thinking leads to a kind of sterile religious behaviorism in which religiosity is measured by outward actions alone, with no thought given to the higher goals of life.
Although it is possible to discuss the mitzvot individually and the reasons for observing them - and the Sages themselves found reasons for the vast majority of them, as did Maimonides and other Jewish philosophers - it is also possible to look for a general principle or purpose that governs the entire system. If that principle is valid, then perhaps it is worth observing the mitzvot regardless of one's point of view.
My late friend Rabbi Hertzel Fishman often quoted the rabbinic statement "The mitzvot were given only in order to purify human beings" (Genesis Rabbah 44) as a justification for observing them today. Thus the entire purpose of observing any of them is in order to improve ourselves, to make us better people - more caring, more loving, more concerned. The mitzvot are not magic, they do not help God nor do they earn us rewards, but they are valuable for the way in which they improve our character and thereby benefit the society in which we live.
Rabbinic literature also reflects this desire to find an underlying principle behind the way of mitzvot. Rabbi Simlai, a 3rd century Amora, taught that David reduced the 613 mitzvot to 11, Isaiah to six, Micah to three (do justly, love mercy and walk humbly before Your God [6:8]), Isaiah again to two (keep justice and do righteousness [56:1]) and finally Amos to one, "Seek Me and live" (5:4) (Makkot 23b-24a). Simlai was not saying that we did not need to observe the 613 mitzvot, something that Pauline Christianity had asserted, but rather that all the mitzvot were to be seen as an attempt to bring justice and righteousness into the world. They are our way of truly seeking God. The 613 mitzvot are details of a larger philosophy, a greater belief, intended to improve the character of each person.
That which distinguishes Judaism as a religious way of life is the emphasis on deeds rather than on abstract belief. This does not denigrate the importance of belief, indeed a belief in God as One and as concerned with morality is the basis of Judaism, but these beliefs must be translated into specific deeds. Moreover, actions are the way in which character is molded.
Another attempt to find the basic principle underlying the Torah, the principle behind all the mitzvot, was made earlier by the great Rabbi Akiba. He said that "Love your neighbor as yourself" (Leviticus 19:18) was the basic principle (Genesis Rabbah 24:7). In this teaching, Akiba followed his predecessor, Hillel the Elder, who told a non-Jew who was willing to convert if Hillel could teach him all the Torah while standing on one foot, "That which is hateful to you, do not do to your fellow. That is all the Torah. The rest is commentary" (Shabbat 31a). This was a negative Aramaic formulation of the verse "Love your neighbor as yourself." The late chancellor of the Jewish Theological Seminary, Rabbi Louis Finkelstein, explained that by this Akiba and Hillel meant that all the mitzvot had but one purpose: to produce a person who would love his neighbor.
The real test of one's religiosity then, is not so much in the scrupulous observance of each detail, or in the numerical adding up of how many mitzvot one has observed, but in the way in which this observance has changed the individual. Are we "purified" by our observance, more loving of our fellow? If so, the performance of the mitzvot has achieved its purpose. If not, it has been in vain. We sell Judaism short when we fail to recognize that it is not a mere legal system or a list of dry behavior patterns, but rather an organized system of living designed to bring human beings to a height of moral perfection that will permeate every aspect of life.