"For once they used it at less secret gate, Which finds itself without a fastening still"

(Canto 8, lines 125-126)

This is commonly taken to refer to the time when Jesus descended into Limbo to rescue certain souls as described in Canto 4: he would then have run into patrol at the outer gate of Hell (the "less secret gate") which ever since he broke through the devil's ranks has been "without a fastening".