In the Pure Land school…[w]e emulate Avalokitesvara ’s compassion and Mahasthamaprapta's single-mindedness. The "Chapter of the Perfect and Complete Realization of Mahasthamaprapta" in the Surangama Sutra tells us how Mahasthamaprapta and fifty-two fellow bodhisattvas single-mindedly concentrated on being mindful of Amitabha Buddha and on chanting his name from the time they took refuge until the time they attain Buddhahood. All they relied on was the name of Amitabha Buddha—[as the sutra says] "without the aid of any other expedient, the mind will be opened."
In other words, Pure Land practitioners do not need to rely on any other method. From their initial determination to seek enlightenment until their attainment of Buddhahood, they only need to concentrate on nianfo—on being mindful of Amitabha Buddha and on chanting his name.
How should we practice nianfo? Mahasthamaprapta taught us that the key is "the perfect control of the six senses with continuous pure thoughts." "Pure thoughts" means that when we chant "Amituofo" we should not have any doubts or other thoughts. The mind must be pure; we must chant with a pure mind. "Continuous" means that we chant with no interruption. "No intermingling, no doubt, and no interruption" is the key to success in the practice of nianfo that Mahasthamaprapta taught us. If we practice "the perfect control of the six senses with continuous pure thoughts," we will definitely succeed in our practice of nianfo.
~ Venerable Master Chin Kung