In the Gospel reading from Mark 3, Jesus is accused of being possessed, and driving out demons by the prince of demons.
Jesus replies by talking about a kingdom divided against itself. He also says, "But no one can enter a strong man's house to plunder his property unless he first ties up the strong man. Then he can plunder his house."
Spiritually speaking, Satan wants to plunder your house. Jesus intimates that that would be difficult to do to one who is strong. Conversely, it would be easy to do to one who is weak (in faith). We must remain strong, that our house (personal and corporate - i.e. the Church) may not be plundered. So how do we do that? Well, just as someone becomes stronger by working their muscles, so it is in the spiritual life - we must work out our spiritual muscles. Pray, go to confession, participate in the sacramental life of the Church, serve others. All of these will make us stronger, and less susceptible to Satan's plundering ways.
Father, give us the grace to work out our spiritual muscles, that Satan may have no victory over us. Amen.
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