When anything truly glorious is done, there ostentation has its readiest occasion; so the Lord first shuts out all intention of seeking glory, as He knows that this is of all fleshly vices the most dangerous to man. Observe how He has begun as it were describing some beast hard to be discerned, and ready to steal upon him who is not greatly on his guard against it; it enters in secretly, and carries off insensibly all those things that are within. It is our heart we must watch, for it is an invisible serpent that we have to guard against, which secretly enters in and seduces; but if the heart be pure into which the enemy has succeeded in entering in, the righteous man soon feels that he is prompted by a strange spirit; but if his heart were full of wickedness, he does not readily perceive the suggestion of the devil.
When almsgiving, 'Let not your left hand know what your right hand is doing,' is said as an extreme expression, as much as to say, If it were possible, that you should not know yourself, and that your very hands should he hidden from your sight, that is what you should most strive after. If therefore you desire spectators of your good deeds, behold you have not merely Angels and Archangels, but the God of the universe.
It is a good thing to be drawn away from the thought of empty glory, but especially in prayer. For our thoughts are apt to stray of themselves; if then we address ourselves to prayer with this disease upon us, how shall we understand those things that are said by us? Our chambers are to be understood our hearts of which it is spoken in the fourth Psalm: What things you utter in your hearts, and wherewith you are pricked in your chambers (Ps 4:4). The door is the bodily senses; without are all worldly things, which enter into our thoughts through the senses, and that crowd of vain imaginings which beset us in prayer. The door then must be shut, that as we must resist the bodily sense, that we may address our Father in such spiritual prayer as is made in the inmost spirit where we pray to Him truly in secret.
Homily 19 on Matthew
Opus Imperfectum in Matthaeum (Pseudo-Chrysostom)
On the Sermon of the Mount, Bk. 2, Ch. 3 (St. Augustine)