Jesus does not say you cannot very well serve God and mammon.
Rather, He says you cannot serve two masters at all.
If you try, you will surely end up serving only one. The person who thinks he or she is serving God a little is deceived.
He or she is not serving God. God will not have that service. The devil will monopolize the person before long.
A divided heart loses both worlds. Saul tried it. Balaam tried it. Judas tried it. Their efforts ended in desperate failure.
For Mary there was but one choice. Paul said: This one thing I do, and For me to live is Christ.
Of such a life God says: Because he hath set his love upon me, therefore will I deliver him: I will set him on high, because he hath known my name (Psalm 91:14).
God takes a peculiar pride in revealing His love to the heart that wholly chooses Him.
Heaven and earth will fade away before its trust can be disappointed.
Have we chosen Him only and given Him all our hearts?
Say, is it all for Jesus, As you so often sing? Is He your Royal Master? Is He your heart's dear King?