"And God spoke all these words saying... You shall have no other gods before me. You shall not make for yourself a carved image, or any likeness of anything that is in heaven above, or that is in the earth beneath, or that is in the water under the earth. You shall not bow down to them or serve them, for I the LORD your God am a jealous God..." Exodus 20.
"You shall have no other gods.
What this means: You shall have Me alone as your God...
What is God? A god means that from which we are to expect all good and in which we are to take refuge in all distress. So, to have a God is nothing other than trusting and believing Him with the heart... If your faith and trust is right, then your god is also true. On the other hand, if your trust is false and wrong, then you do not have the true God. For these two belong together, faith and God [Hebrews 11:6]. Now, I say that whatever you set your heart on and put your trust in is truly your god.
The purpose of this commandment is to require true faith and trust of the heart, which settles upon the only true God and clings to Him alone. It is like saying, 'See to it that you let Me alone be your God, and never seek another.' In other words, 'Whatever you lack of good things, expect it from Me. Look to Me for it. And whenever you suffer misfortune and distress, crawl and cling to Me. I, yes, I, will give you enough and help you out of every need. Only do not let your heart cleave to or rest on any other...'
Many a person thinks that he has God and everything in abundance when he has money and possessions. He trusts in them and boasts about them with such firmness and assurance as to care for no one. Such a person has a god by the name of 'Mammon' (i.e., money and possessions; [Matthew 6:24]), on which he sets all his heart. This is the most common idol on earth. He who has money and possessions feels secure [Luke 12:16-21] and is joyful and undismayed as though he were sitting in the midst of Paradise. On the other hand, he who has no money doubts and is despondent, as though he knew of no God. For very few people can be found who are of good cheer and who neither mourn nor complain if they lack Mammon. This care and desire for money sticks and clings to our nature, right up to the grave.
So, too, whoever trusts and boasts that he has great skill, prudence, power, favor, friendship, and honor also has a god. But it is not the true and only God. This truth reappears when you notice how arrogant, secure, and proud people are because of such possessions, and how despondent they are when the possessions no longer exist or are withdrawn. Therefore, I repeat that the chief explanation of this point is that to 'have a god' is to have something in which the heart entirely trusts...
So you can easily understand what and how much this commandment requires. A person's entire heart and all his confidence must be placed in God alone and in no one else. For to 'have' God, you can easily see, is not to take hold of Him with our hands or to put Him in a bag <like money> or to lock Him in a chest <like silver vessels>. Instead, to 'have' Him means that the heart takes hold of Him and clings to Him. To cling to Him with the heart is nothing else than to trust in Him entirely. For this reason God wishes to turn us away from everything else that exists outside of Him and to draw us to Himself [John 6:44]. For He is the only eternal good [Matthew 19:17]. It is as though He would say, 'Whatever you have previously sought from the saints [or anyone else], or for whatever things you have trusted in money or anything else, expect it all from Me. Think of Me as the one who will help you and pour out upon you richly all good things.'
See, here you have the meaning of the true honor and worship of God, which pleases God, and which He commands under penalty of eternal wrath. The heart knows no other comfort or confidence than in Him. It must not allow itself to be torn from Him. But, for Him, it must risk and disregard everything upon earth... If your heart does not take refuge in Him but flees from Him when in trouble, then you have an idol, another god..."
"'Which commandment is the most important of all?' Jesus answered, 'The most important is, 'Hear, O Israel: The Lord our God, the Lord is one. And you shall love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.'" Mark 12:28-30