What "religions" did previous Buddhas practice? I recently found a book called "A Buddhist Bible".

In it, Buddha refers to that there have been many Buddhas before him.
If Buddha was the one to put the foundations of Buddhism, what "religions" did the previous Buddhas practice?

Does this question even make any sense?
According to tradition, as in the Nagara Sutta, Buddhas all discover the path to enlightenment and teach that same path to their followers:


  "It is just as if a man, traveling along a wilderness track, were to
  see an ancient path, an ancient road, traveled by people of former
  times. He would follow it. Following it, he would see an ancient city,
  an ancient capital inhabited by people of former times, complete with
  parks, groves, &amp; ponds, walled, delightful. He would go to address the
  king or the king's minister, saying, 'Sire, you should know that while
  traveling along a wilderness track I saw an ancient path... I followed
  it... I saw an ancient city, an ancient capital... complete with
  parks, groves, &amp; ponds, walled, delightful. Sire, rebuild that city!'
  The king or king's minister would rebuild the city, so that at a later
  date the city would become powerful, rich, &amp; well-populated, fully
  grown &amp; prosperous.
  
  "In the same way I saw an ancient path, an ancient road, traveled by
  the Rightly Self-awakened Ones of former times. And what is that
  ancient path, that ancient road, traveled by the Rightly Self-awakened
  Ones of former times? Just this noble eightfold path: right view,
  right aspiration, right speech, right action, right livelihood, right
  effort, right mindfulness, right concentration. That is the ancient
  path, the ancient road, traveled by the Rightly Self-awakened Ones of
  former times. I followed that path. Following it, I came to direct
  knowledge of aging &amp; death, direct knowledge of the origination of
  aging &amp; death, direct knowledge of the cessation of aging &amp; death,
  direct knowledge of the path leading to the cessation of aging &amp;
  death. I followed that path. Following it, I came to direct knowledge
  of birth... becoming... clinging... craving... feeling... contact...
  the six sense media... name-&amp;-form... consciousness, direct knowledge
  of the origination of consciousness, direct knowledge of the cessation
  of consciousness, direct knowledge of the path leading to the
  cessation of consciousness. I followed that path.
  
  "Following it, I came to direct knowledge of fabrications, direct
  knowledge of the origination of fabrications, direct knowledge of the
  cessation of fabrications, direct knowledge of the path leading to the
  cessation of fabrications. Knowing that directly, I have revealed it
  to monks, nuns, male lay followers &amp; female lay followers, so that
  this holy life has become powerful, rich, detailed, well-populated,
  wide-spread, proclaimed among celestial &amp; human beings."


So, all Buddhas essentially practice and preach the same religion.