You shall have no other gods before me.

Ex. 20: 3.

The Jews abstained outwardly from idolatry, but their hearts
were far from God, full of mistrust and unbelief. Outwardly
they appeared beautiful, as though they meant it in all sincerity,
but within they were full of idolatry. They understood this
law in Exodus to mean no more than that they should not set up idols and
images to worship, and when they could confess with their lips
that they have only one God and honor no other gods, they
thought they had kept this commandment.

Now we must have high regard for the law. You, it
says, You, and everything you are; it means especially the
heart, the soul, and all the powers. It does not speak of the
tongue, or the hands, or the knees; but it speaks of the whole
body and of all you have and are. If I am to have no other
God, then I must surely possess the only true God with my
heart, I must in my heart be affectionate to him, evermore cleave
to him, depend upon him, trust him, have my desire, love and
joy in him, and always think of him. You shall love God
with all your heart, so that your whole body from the crown of
the head to the soles of the feet, inwardly and outwardly, goes
forth in love, rejoices in God and honors him.

Now find me a man who is pure and otherwise pious with
a burning passion and love; there is none such on the earth.
We find ourselves much more inclined to anger, hatred, envy,
worldly pleasures. You see, you poor condemned creature, you
should have delighted in God's law, and yet you had no pleasure
in it. Had we now remained in this condemnation, we would
have had to perish forever. Therefore Christ comes and offers
his mercy, and says: You are to love God with all your heart,
but you have not done it; now believe in me. Then when we
come before God the Father, Christ will say: Father! Although
they have not wholly fulfilled your law, yet I have done so, let
this be to their benefit because they believe in me.