Who left his house - The word “house” often means family. Our Saviour here represents himself as going away, leaving his household the church, assigning to the apostles and all his servants their duty, and leaving it uncertain when he would return. Since his return was a matter of vast consequence, and as the affairs of his kingdom were entrusted to them, just as the affairs of a house are to servants when the master is absent, so it was of vast importance that they should be faithful at their post, that they should defend the house from danger, and be ready for his return.
The porter - The doorkeeper. To the janitor or doorkeeper was entrusted particularly the care of the house, whose duty it was to attend faithfully on those who came and those who left the house.