Awakening to our true nature or pure Mind first entails a brief, sudden glimpse. You see it. That’s it! It is somewhat like looking for gold in an old abandoned mine. One day the metal detector you are using picks up a trace of gold which appears to be the beginning of a vein of gold. This is a sudden affair. It’s exciting. Before you didn’t see the gold, but were looking, now you see it. Next, the hard work begins. You have to get the gold out of the vein which will require a lot of hard work.
To reiterate, seeing our true nature is all of a sudden. But once we see it we have to keep connecting with it, again and again. This is samadhi which entails connecting with this pure nature as much as possible. As we connect with it, more and more, we become more of it and less of our former self. Zen Master Shen-hui (神会) put it this way:
“All those who want to learn the Tao (Way) must achieve Sudden Enlightenment to be followed by Gradual Cultivation. It is like child-birth, which is a sudden affair, but the child will require a long process of nurture and education before he attains his full bodily and intellectual growth."
By the repetition of connecting with pure Mind, this process being samadhi, we come successively closer to becoming Buddhas; thus becoming fully awakened instead of partially. Our earlier brief glimpse after so many years bears much fruit. The toddler becomes a teenager and eventually an adult and finally a wise old dude!
Sudden and gradual are very much coordinate. They don’t oppose or go against each other as we are sometimes led to believe. There is no ‘versus’, in other words.