Deuteronomy 31:6 Be strong and of a good courage, fear not, nor be afraid of them: for the Lord thy God, he it is that doth go with thee; he will not fail thee, nor forsake thee.
Psalm 118:6 The Lord is on my side; I will not fear: what can man do unto me? 7 The Lord taketh my part with them that help me: therefore shall I see my desire upon them that hate me.
Conclusion: The holy spirit wrote the letter to the Hebrews through one or more of His servants well versed in the Greek language.