Who was known as 'Constantine of Buddhism'?

Emporer Ashoka

* Asoka send royal officers out to teach Buddhism. So he called Constantine of Buddhism. If you meant Asoka as in the grandson of Chandragupta Maurya: He stop war when his son died. He then decided to take up buddhism and spread buddhism around India. He converted to Buddhism after witnessing many deaths in the war of kalinga. He sent messengers to many countries e.g. Sri Lanka, to teach people about Buddhism.