[Ali Imran]

Hebrews 6:4-6
4 It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, 
5 who have tasted the goodness of the word of God and the powers of the coming age 
6 and who have fallen away, to be brought back to repentance. To their loss they are crucifying the Son of God all over again and subjecting him to public disgrace.

Some NT scholars opined that the book of Hebrews was written by St Paul. 

Do you know that even St Paul didn't teach the Trinity? That means, even St Paul didn't know the Trinity.