Read: Jeremiah 17:1-18
Idolatry is to have some other object of trust or worship instead of, or along with, the One True God who has revealed Himself in the Scripture.
If Jehovah is God alone, why does He bother Himself and us with the problem of idolatry?
There are several reasons. First, because he is God alone, He has the right to demand the exclusive worship of all mankind, and to be jealous for the honor of His own name (Exo 34:14). This is a perfectly reasonable expectation. How wrong of us not to give Him the honor that is His due.
Second, God concerns Himself with idolatry because man's sinful heart loves to invent false gods to take the place of the True God. Man in his sinfulness wants an escape from God, wants a God who is permissive, who will let him be in his sin, but will be there when he's in trouble. In this commandment, God addresses man's fundamental problem, and puts before him his basic calling and purpose. On the one hand, man's problem is selfishness; he wants to serve self, and he wants the world to be ordered to his advantage, while, on the other hand, man's reason for existence is to bring glory to God.
Third, even though these gods are false, they have the power to enslave man's heart. When a man puts his confidence in a creature, Satan is busy, attempting to steal that person's allegiance from God to himself. Ultimately, all those who do not worship the true God of the Scriptures are giving their allegiance and worship to the devil. 1 Cor 10:20 tells us that the things offered to idols are “sacrifice[d] to devils, and not to God”.
The first commandment establishes Christianity as an either-or, not a both-and religion. When it comes to religion and worship of God, there is no middle ground. There can be no synthesis of Christianity and the other religions. When Jesus told his disciples, “No man can serve two masters” and “Ye cannot serve God and mammon” (Matt 6:24), He was simply reminding them of the first commandment.
Worshiping Jehovah is like a marriage. When you marry a woman, you do it to the exclusion of all other women. You do not say to your wife, “Here is my other lover, I'd like you to meet her so that you can become friends.” Your wife will say, “It's me or her, you pick,” and no one would fault her for that. “Forsaking all others” we cleave to one another in marriage. The same is true in our relationship to God. He is jealous of our love.