Spirituality
In accordance with Gene Roddenberry's Humanist vision, none of the TOS characters ascribe to religion. False gods are a recurring motif. However, it's often been observed that the characters unfailingly behave like white Protestants, regardless of their diverse backgrounds. The exception is - who else? - Spock.
We know the Vulcan is not a does not practice any one of Earth's religions; we can't be sure if Earth's people do in the 23rd century! Still, his beliefs and behaviors seem derived the faiths of the near and far east. This distinguishes him from American Midwesterners like Kirk and McCoy, and gives him an air of strangeness and mystery. Both in Spock's life and in his planet's culture, we see nods to the Orient. Here we look at several traditions, to see how each of these influences our Vulcan.
Judaism. This is the most obvious influence. Leonard Nimoy, we know, is a prominent member of the Jewish community, and has incorporated components of this ancient culture into Spock's character. The most widely known is his adoption of a rabbi gesture of blessing, the Shin, as the Vulcan hand salute. We know that Spock is a strict vegetarian, which might be traced to the Jewish dietary laws as much as '60s grassroots movements.
Spock is certainly not an Orthodox Jew. (Nor "orthodox" anything.) Rather, he takes cues from Jewish mysticism, or Kabbalah. Nimoy has long been fascinated with this tradition, and in the photo collection Shekhina he explores it with an emphasis on God's feminine incarnations. Spock, meanwhile, acts with kindness and moderation, meditates, and generally probes the secrets of the universe with more than scientific instruments. His contact with V'Ger in The Motion Picture is certainly a spiritual experience, accomplished through Spock's Vulcan abilities.
Vulcan mysticism is "Hollywood occult" to an extent, but also resemble Kabbalah. The Vulcans' telepathy and physical prowess are similar the fantastic powers of rabbis in ancient texts, and there is an emphasis on ritual and the direction of one's energies to a task. (As we see during the fal-tor-pan in The Search for Spock.)
However, the Jewish belief in a diety is not something the Vulcans share; to further understand their values, we must go further east.
Buddhism. This faith is most strongly associated with meditation, which Spock practices. This exercise requires and promotes concentration and the direction of the mind to a single point. While for humans this sharpens the senses and promotes health, for Spock it has some more tangible results. We are told his great mental discipline allows him to practice the Vulcan mind meld; not every Vulcan can. He consistently demonstrates quick and clear thinking, and catches details his fellow officers overlook. Even off-duty, he is keenly aware of sensory experiences. (In music, food, etc.)
The Vulcan kolinar is similar to nirvana: both aim to free one from the bonds of desire. However, the Vulcans seek to terminate their passion, while Buddhists seek to experience bliss. Spock does not succeed at the kolinar, since his feelings call him elsewhere. While unable to meet the rigorous demands of the Vulcans, he would have a fine Buddhist.
Bahá'í Faith. If one had to confer a single religion on Spock, it would probably be the Bahá'í faith. His allegiance to Infinite Diversity in Infinite Combinations or IDIC is the first piece of evidence. Bahá'ís believe in an essential unity of all the Earth's religions but recognize their distinct cultural aspects. Just as Spock is an advocate for justice and equality, even in an era that has supposedly achieved it, so Bahá'ís were ahead of their time in promoting race and gender equality, a world government (as maintained in TOS and through the Federation), and freedom and peace.
The most singular belief lies in an individual, immortal soul, that is free to travel after the body's death. This is remarkably similar to the Vulcan katra; again, what is a spiritual view in our world becomes a literal truth in a science fiction universe. Spock would find much to like in Bahá'í and would confirm their conception of the soul.
Christianity. While the influence of the world's largest religion is less marked in Spock than his shipmates, it can't be entirely ignored. Spock did the "death and resurrection" thing long before it became an action movie staple, and exhibits some other vaguely Christ-like characteristics. (Born to a human mother and unearthly father, and the Jewish connection.) Still, Spock is most "Christian" in his sense of fellowship. He admits more than once that his family is Kirk and McCoy, and through the institution of the Federation, these three help many others. Kirk's belief that God is in the human heart is in keeping with liberal Protestant beliefs. Spock has a difficult time accepting this simple notion. He takes more solace in his own broadly influenced and unique spirituality.