Nicole, before you have your sit down with Prothero please check out this review by Georgetown Theology prof Leo D. Lefebure:
http://www.christiancentury.org/article.lasso?id=8479
Lefebure is very sympathetic to Prothero’s point of view, but feels that Prothero simply fails to make a good case (to put it mildly).
Another very thoughtful and substantial critical was written by Alec Solomita for the Boston Globe on Sunday May 23:
http://www.boston.com/ae/books/articles/2010/05/23/an_attempt_to_debunk_the_idea_that_all_religions_are_brothers/
The punchline of Solomita’s review is “Yet, despite his promise that he will look at the great religions critically and comparatively, Prothero avoids honest contrasts through a series of logical contortions that make him look as if he’s playing the game Twister.”
Most of the reviews and interviews connected with Prothero’s recent book have been puff-pieces that appear to be arranged by his publicist and/or publisher — in essence, advertisements.

I imagine something like cognitive dissonance is involved. So the more invested you are in your religion the more difference you see between it and others. Conversely, the less invested probably see more similarities.
Basically how sure are you that you’re right? If you are really sure, then others must be wrong in some way. To say that they are similar to you is fighting words.

I am definitely not religion savvy – and to some I might be considered “religion illiterate” – but I do see similarities amongst different religions … could it be that I am less invested in my own religion?? Perhaps!

Stephen Prothero says that he believes in the Christian Trinity, but that other religions do not. There are “trinities,” of sorts, in various faiths. The e-book at http://www.suprarational.org summarizes five of them.
Mahayana and Vajrayana vehicles of Buddhism speak of Trikaya, or three bodies: Nirmanakaya is the Buddha in human form, Sambhogakaya is celestial Buddha and Dharmakaya is the formless essence, or Buddha-nature. The Theravada primarily addresses the historic Buddha. The “Three Jewels” are the Buddha, the dharma (his teachings) and the sangha (the community of monks and nuns).
Christianity has its Trinity: Father, Son and Holy Spirit referring to God, Jesus Christ and their spiritual bond of unity (some say the Godhead). Interpretation of the essential nature of each, and their relationship, differed among the churches. In Christian mysticism, the three ways of the spiritual life are the purgative in being purified from sin, the illuminative in true understanding of created things, and the unitive in which the soul unites with God by love.
Hinduism’s trimurti are the threefold activities of Brahman: in Brahma as creator, in Vishnu as sustainer and in Shiva as destroyer. Saccidananda are the triune attributes or essence of Brahman: sat, being, cit, consciousness and ananda, bliss. The three major schools of yoga are bhakti, devotion, and jnana, knowledge and karma, the way of selfless action. Raja yoga can apply to, and integrate, all three in mental and spiritual concentration.
In Islam, nafs is the ego-soul, qalb is heart and ruh is spirit. Heart is the inner self [soul], hardened when it is turned toward ego and softened when it is polished by dhikr, remembrance of the spirit of Allah. This is a three-part foundation for Sufi psychology. Initiation guides them from shari`a, religious law, along tariqa, the spiritual path, to haqiqa, interior reality. It is a gradual unveiling of the Real.
In the Kabbalah of Judaism, sefirot – sparks from the divine – have three fulcrums to balance the horizontal levels of the Tree of Life: Da`at (a pseudo-sefirot) is knowledge combining understanding and wisdom; Tiferet is beauty, the midpoint of judgment and loving kindness; Yesod is the foundation for empathy and endurance. They also vertically connect, through the supreme crown, the infinite and transcendent Ein Sof with its kingdom in the immanent Shekhinah.