If you mean what I think you mean by that phrase, I'd say the "ABCs of religion" would be the Arabic Hidden Words based on the introduction of that particular book:
"This is that which hath descended from the realm of glory, uttered by the tongue of power and might, and revealed unto the Prophets of old. We have taken the inner essence thereof and clothed it in the garment of brevity, as a token of grace unto the righteous, that they may stand faithful unto the Covenant of God, may fulfill in their lives His trust, and in the realm of spirit obtain the gem of divine virtue."
I appreciate your help. Someone told me this phrase "ABC of religion" as by Abdu'l-Baha and possibly in the Star of the West. I have not been able to download the software for Star of the West to check it out. Do you have a link for it?