# that if the master of the house ... broken into

Jesus uses a parable of a master and servants to illustrate that his disciples should be prepared for his return. 

# the thief

Jesus is saying he will come when people are not expecting him, not that he will come to steal. 

# he would have been on guard

"he would have guarded his house"

# would not have allowed his house to be broken into

"would not have allowed anyone to get into his house to steal things"