No one can serve two masters
It is clear then. Those who make Mammon their master cannot serve God, but they serve the master they have freely chosen. They love to be at Mammon's disposal, they are happy to serve him because they have chosen Mammon, because they have voluntarily subjected themselves to him. Generally people love the masters they have chosen of their own free will more than those to whom they have become subject by compulsion.
A different pattern of behaviour is exhibited by a small number of people who are pleasing to God. They have become the masters of their own wealth. And they have used it, as if it were their faithful slave, to feed the hungry, to clothe the naked, to free the debtors who are insolvent and in prison. Abraham, Isaac, Jacob, Job, Joseph, David acted like this. Money was not the master of these people; they were the masters of their money.
'You cannot serve God and Mammon.' Here then is the commandment to be observed by those who possess riches. Christ, seeing that they were not raising themselves to the height of perfection, came down to their level with his word and establish a law which they can keep in the situation in which they are.
In other words, it is as if he had said: 'Seeing that you are not willing to renounce wealth, at least don't become its slave. Become in actual fact its master and use it for all the very best of actions.'
Philoxenus of Mabbug
Homily 8, 226ff (SC44 pp.225ff)