Whether in the past, future, or present, all fully awakened to things as they truly are do so by fully awakening to the Four Noble Truths and making them known.

Dutiyasamaṇabrāhmaṇa sutta - Ascetics And Brahmins (Second)

"Bhikkhus, whatever ascetics or brahmins in the past fully awakened to things as they truly are, all of them fully awakened to the Four Noble Truths as they truly are and made them known. Whatever ascetics or brahmins in the future will fully awaken to things as they truly are, all of them will fully awaken to the Four Noble Truths as they truly are and will make them known. Whatever ascetics or brahmins at present fully awaken to things as they truly are, all of them fully awaken to the Four Noble Truths as they truly are and make them known.

What are the four? The Noble Truth of suffering, the Noble Truth of the arising of suffering, the Noble Truth of the ending of suffering, the Noble Truth of the way of practice leading to the ending of suffering. Bhikkhus, whatever ascetics or brahmins in the past fully awakened to things as they truly are, all of them fully awakened to these Four Noble Truths as they truly are and made them known. Whatever ascetics or brahmins in the future fully awaken to things as they truly are, all of them will fully awaken to these Four Noble Truths as they truly are and make them known. Whatever ascetics or brahmins at present fully awaken to things as they truly are, all of them fully awaken to these Four Noble Truths as they truly are and make them known.

Therefore, bhikkhus, effort should be made to fully understand: 'This is suffering'; effort should be made to fully understand: 'This is the arising of suffering'; effort should be made to fully understand: 'This is the ending of suffering'; effort should be made to fully understand: 'This is the way of practice leading to the ending of suffering.'"