Not everyone can see the tokens of God's grace in the lives of the saints, especially the newer saints. He saw living embers of grace that he wanted to fan into flame, where others saw only the ashes of imperfection. Because faith lives by grace. Faith spends all of its time looking for grace and locking eyes with grace. And so faith has a kind of homing device for grace.
Faith is like a radar screen designed to pick up the slightest motion of grace.
Its like one of those little metal detectors that men use to hunt coins around Lake Calhoun. If it gets near to a tiny little fragment of grace, the signals go off. So Barnabas had the goodness of being able to see the grace of God in the imperfect lives of the Gentile saints in Antioch, because his faith was tuned in to the frequency of grace. Because faith can't survive without grace. Barnabas not only saw the grace of God, he rejoiced over the grace of God in the lives of others.