Daily Dose of Calvin III

The expression, being not weak in faith, take in this sense — that he vacillated not, nor fluctuated, as we usually do under difficult circumstances. There is indeed a twofold weakness of faith — one is that which, by succumbing to trying adversities, occasions a falling away from the supporting power of God — the other arises from imperfection, but does not extinguish faith itself: for the mind is never so illuminated, but that many relics of ignorance remain; the heart is never so strengthened, but that much doubting cleaves to it. Hence with these vices of the flesh, ignorance and doubt, the faithful have a continual conflict, and in this conflict their faith is often dreadfully shaken and distressed, but at length it comes forth victorious; so that they may be said to be strong even in weakness. - John Calvin, commenting on Romans 4:19