Because of the verb "feel", "will" is more natural, but I would accept "would".
Student or Learner
I will/would be happy if you feel that Daishonin actually wrote this letter in order to guide not only Toki Jonin, one of the believers, but also all believers in the world towards the correct path.
(Daishonin is the founder of Nichiren Shoshu Buddhism.)
Should I use 'will' or 'would'?
That is true, but both are far less natural than the simpler "I hope" or even "I would hope..." (the latter being less forceful and therefore more polite -- also more in harmony with Nichiren's religious teachings.