2 “I will pour out before Him my prayer”. What is, “before Him”? In His sight. What is, in His sight? Where He sees. But where does He not see? For so do we say, 'where He sees,' as though somewhere He sees not. But in this assemblage of bodily substances men too see, animals too see: He sees where man sees not. For your thoughts no man sees, but God sees. There then pour out your prayer, where He alone sees, who rewards. For the Lord Jesus Christ bade you pray in secret: but if you know what “your closet” is, and cleansest it, there you pray to God.
“But you,” says He, “when you pray, enter into your closet, and shut the door, and pray to your Father in secret, and He who sees in secret shall reward you.” If men are to reward you, pour out your prayer before men: if God is to reward you, pour out your prayer before Him; and close the door, lest the tempter enter. Therefore the Apostle, because it is in our power to shut the door, the door of our hearts, not of our walls, for in it is our “closet,”— because it is in our power to shut this door, says, “neither give place to the devil.” But what is to “shut the door”?
This door has as it were two leaves, desire and fear. Either thou desires something earthly, and he enters by this; or you fear something earthly, and he enters by that. Close then the door of fear and desire against the devil, open it to Christ. How do you open these folding doors to Christ? By desiring the kingdom of heaven, by fearing the fire of hell. By desire of this world the devil enters, by desire of eternal life Christ enters; by fear of temporal punishment the devil enters, by fear of everlasting fire Christ enters....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)