What does it mean to “become a Buddha” especially after one dies since we say that we all become Buddhas once we die?
To “become a Buddha” means to become Enlightened. In our tradition of Shin Buddhism (Jodo Shinshu), it is the workings of Amida Buddha that allows us to fulfill our full potential and to become a Buddha, either in this life or most certainly after we die.
It is the unique Teachng of Shinran, the founder of Shin Buddhism, who showed us the Path of Awakening in this life so we can have the benefit of Enlightenment while we are still alive which is the core of Buddhism.
One cannot use logic to define Enlightenment because a finite human being cannot comprehend the Infinite. Thus, to be Enlightened is doing the impossible and can only be understood in a spiritual sense.
The historical Shakyamuni Buddha was the first known human being to become a Buddha.The word Buddha is a title that means the Enlightened One.