In Joshua 1:5 God said to Joshua, “As I was with Moses, so will I be with you; I will not fail you, nor forsake you.” In Hebrews 13:5 the writer said, “Let your conduct be without covetousness; and be content with the things that you have: for he hath said, I will never leave you, nor forsake you” quoting the promise to Joshua. How did he get from a specific promise made to an Old Testament individual, to a general promise for all New Testament believers? Was he applying a spiritual principle or describing a characteristic of God?
I believe the writer was referring to a characteristic of God that is evident through out the Old and New Testaments. For example, in Matt. 28:20 Jesus said, “And surely I am with you always, even to the end of the age.” Although He was speaking to the disciples, he obviously had the entire Church Age in mind. Hebrews could have been written before Matthew, which would explain why the writer used the Old Testament quote instead of the one from Matthew.