Albert Barnes on Mark 13:34

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. As 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, defend the house from danger, and be ready for his return.

The porter. The door-keeper. To the janitor or door-keeper was entrusted, particularly, the faithful care of the house, whose duty it was to attend faithfully on those who came, and those who left the house.