It seems to me that the Bible does clearly distinguish different levels of commitment. There is the somewhat perplexing text in John 2:23-25, which speaks of those who “believed in our Lord’s name” to whom Jesus would not “commit himself,” because He “knew what was in man.”
In the Gospels, Jesus distinguishes between faithful and unfaithful servants (though in some contexts at least, the unfaithful servants may well be unbelieving Jewish religious leaders, who have abused their stewardship).
In Luke chapter 12:47-48 our Lord speaks of differing degrees of punishment, and I think we can also conclude that there are differing levels of reward (Matthew 25:21ff.; Luke 19:17). In 1 Corinthians 3:10-15 we read of the rewards we will receive as a result of “how we have built on the foundation, which is Jesus Christ.” Paul ends this by saying there are those who will be saved, “yet so as by fire” (verse 15). Paul tells Timothy to commit himself to “faithful men” (2 Timothy 2:2), which implies that there are “unfaithful men” as well. In Galatians Paul speaks of those who “are spiritual” (6:1) who are to restore one who is caught in some sin. In Revelation there are the “overcomers.”
All of this is to say that distinctions are made between believers.