Faith, Vows and Practice
These three factors are the cornerstones of Pure Land Buddhism. If they are present, rebirth in the Pure Land is achieved.
Faith means faith in Amitabha Buddha's Vows to rescue all who recite His name, as well as faith in one's own Self-Nature, which is intrinsically the same as His (to recite the Buddha's name is to recite the Mind).
Vows are the determination to be reborn in the Pure Land - in one's pure mind - so as to be in the position to save oneself and others.
Practice generally means reciting the Buddha's name to the point where one's Mind and that of Amitabha Buddha are in unison - i.e. to the point of singlemindness. Samadhi and wisdom are then achieved.
Please note that all buddhist teachings are expedients, dividing the one and indivisible Truth into many parts.
Faith, Vows and Practice, although three are really one.
Thus it can be said that rebirth in the Pure Land depends on three conditions, two conditions (Faith or Vows) or even one condition (Faith), as the one contains all and all is contained in the one.
The formula to be used depends on the audience and the times. The aim is to enable sentient beings to achieve rebirth in the Pure Land as a steppingstone towards buddhahood.