This one is a bit complex.  On some level, it is at this point in the drama where the tension between Blanche and Stanley is reaching its zenith.  Stanley has been able to assemble what he needs to discredit Blanche in the eyes of her sister, his wife.  It is at this point that Blanche needs to establish some level of social credibility and something redeeming for all to see.  Blanche's line about God comes when Mitch holds and embraces her at the end of the scene.  The fears of her growing old and alone, enduring isolation and a sense of loneliness are allayed when Mitch holds her, giving her for a moment a glimpse of potential happiness and settlement in a life that has not known either in a real sense.  The idea of God and "quickly" brings to light how the divine force of benevolence can grant redemption even before one asks or one feels its presence.  In this case, Mitch's embrace is love and a blessing to Blanche even before she could articulate it.  Like everything else in Williams' writing, there might be an insinuation that undercuts this positive reading.  Blanche's reading of the quick nature of God could also be reflective of the fact that God's presence is fleeting, for just as he is there "so quickly," he leaves with equal speed or flight.  In this light, the presence and absence of God is one that Blanche sees as quick and temporary, a condition that is not transcendent or permanent in a context that is fleeting and momentary.