Was there a time when Christ was not a believer? What a strange question, right?!
And yet, the following translation of Romans 8:29 does sound to me that God’s Son was the first believer:
Those whom God had already chosen he also set apart to become like his Son, so that the Son would be the first among many believers. (GNT/TEV)
BTW, I’m not convinced that “firstborn”, found in several other English translations, is much of an improvement, although it literally translates what the Greek says. One of my sons-in-law asked me about the “firstborn” idea applied to Jesus Christ a couple of days ago. I explained to him that I felt that the word “firstborn” gives the wrong idea to English Bible readers. The meaning I get from “firstborn” is that Christ had a beginning and that he was the first child born to God. Some, of course, do believe that this is the correct meaning, that Christ was a created being and that God had more than one child.
I was taught something different in my theology classes, that Christ, the second person of the Trinity, is just as eternal as God the Father, but most English speakers lack the background to understand what “firstborn” means. If the Greek does not refer to an actual birth, then I don’t think that an English translation should either. We want translations to communicate the original meaning as accurately as possible. We should not have to teach people that the words on the page of the Bible don’t actually mean what we think they mean. Teaching often is necessary, but it should not be necessary to teach people the meanings of words. Teaching should have to do with helping people understand concepts that words alone do not adequately communicate.