Question: How did all Buddhas attain Buddhahood?
Answer: The answer is clear in this teaching by the Buddha:
《佛说观佛三昧海经》 (卷第九, 本行品第八)
‘[Śākyamuni] Buddha told Ānanda, “You [and all listening, should] now virtuously uphold, and be careful not to forget, that in the past and future, all Buddhas of the three periods [of time: past, present and future], all Buddha World-Honoured Ones, all speak thus, on the Samādhi [Meditative Concentration] From Mindfulness Of Buddha. I, and this Auspicious [Bhadra] Kalpa’s [i.e. this era’s] all great Bodhisattvas, due to [practice of] mindfulness of Buddha thus, attain(ed) all-knowing wisdom, and awe-inspiring ease [for guiding all beings also to complete liberation]. Thus did the ten directions’ [i.e. (East, North, West, North, Southeast, Southwest, Northeast, Northwest, Above, Below); everywhere’s] all immeasurable Buddhas, all by this Dharma [practice] accomplished [Anuttarā Samyak] Saṃbodhi [i.e. unsurpassable equally perfect enlightenment of Buddhahood].”‘
– Śākyamuni Buddha
(The Sûtra In Which The Buddha Speaks Of The Ocean Samādhi From Contemplation Of Buddha: Scroll Nine, Chapter Eight: Fundamental Practice)
 Having encountered this teaching, we are now an extension of the audience, who should also pay attention to it, be mindful of it, and practise mindfulness of Buddha accordingly. This is so as it is a universally and timelessly relevant teaching.
 In our more ‘localised’ current context too, all great Bodhisattvas attained, and will attain Buddhahood with the practice of mindfulness of Buddha.
 In fact, all Buddhas of all periods (time) and directions (space) attained Buddhahood through the practice of mindfulness of Buddha.
 Even if there are other mixed Dharma practices along the path to Buddhahood, eventually, the final practice, also as stated in the last chapter of the Avataṃsaka Sûtra (华严经), will be a practice that is a form of mindfulness of Buddha. This is so as Bodhisattvas must ‘keep their eyes on the prize’ of Buddhahood. 84,000 (innumerable) Dharma practices lead to this one final Dharma door for entry to Buddhahood, just as there is one finish line to cross. It is with mindfulness of Buddha that all become Buddhas sooner or later, depending on the depth of practice, which can be perfected in the swiftest way possible through Amituofo’s (Amitabha Buddha’s) Pure Land. It is also with mindfulness of Buddha that we reach it.