Pureland Buddhists are taught that the six syllables of the Nembutsu — Namo Amida Bu — are not a mantra. But the reality is that Namo Amida Bu is indeed a mantra. Not all mantras, however, are the same. In Buddhism alone, there are an abundance of practical mantras that bestow protection, heal, increase wealth, curry favor, confer magical powers (siddhi), control spirits, etc. The Nembutsu is not a mantra like this.
For devotees of Amida Buddha, the Nembutsu is a supreme invocation. It is a heart-full calling out to that which is measureless and unconditional. It is an invocation of the highest truth — that which is beyond BEYOND. It is an impractical mantra. It does not grant earthly boons. It is purely religious in its purpose and for that reason it is ultimately valuable and supreme.
Which is not to say that Namo Amida Bu is the supreme — singular — mantra. It is not. Other traditions have different mantras connected to what theologian Paul Tillich would identify as their ultimate concern. Like the Nembutsu, these mantras are impractical — without practical worldly application. They are an activity of heart and spirit. They are prayer and invocation all tied up together.
Mantras, like all the practical concerns of life, are useful. The Nembutsu, Namo Amida Bu, is not useful or practical and in this sense it is certainly not a mantra.
Peace, Paul