Shemot/Exodus 20:3
Lo yihyeh lecha elohim acherim al-panay.
Do not have any other gods before Me.

This mitzvot is repeated in D'varim (Deuteronomy 5.7)

If you forsake YHVH, and serve foreign gods, then he will turn and do you evil, and consume you, after that he has done you good. The people said to Yehoshua, No; but we will serve YHVH. Yehoshua said to the people, You are witnesses against yourselves that you have chosen you YHVH, to serve Him. They said, We are witnesses. Now therefore put away, [said he], the foreign gods which are among you, and incline your heart to YHVH, the Elohim of Yisra'el. The people said to Yehoshua, YHVH Eloheynu will we serve, and to His voice will we listen. (Yehoshua 24:20-24)

In this passage the people of Yisra'el decided that they would reject the gods of the nations and follow YHVH.

However every nation made gods of their own, and put them in the houses of the high places which the Shomroni had made, every nation in their cities in which they lived. The men of Bavel made Sukkot-Benot, and the men of Kut made Nergal, and the men of Hamat made Ashima, and the `Avvim made Niv'chaz and Tartak; and the Sefarvayim burnt their children in the fire to Adrammelekh and `Anammelekh, the gods of Sefarvayim. So they feared YHVH, and made to them from among themselves Kohanim of the high places, who sacrificed for them in the houses of the high places. They feared YHVH, and served their own gods, after the manner of the nations from among whom they had been carried away. To this day they do after the former manner: they don't fear YHVH, neither do they after their statutes, or after their ordinances, or after the Torah or after the mitzvah which YHVH commanded the children of Ya`akov, whom he named Yisra'el; with whom YHVH had made a covenant, and charged them, saying, You shall not fear other gods, nor bow yourselves to them, nor serve them, nor sacrifice to them (M'lakhim Bet /2 Kings17.29-35)

In the above passage, we see that the people tried to have it both ways. They serverd YHVH, but served their own gods too.They tried to mix the worship of the gods of the nation with the worship of YHVH. Notice in particular what it says in verse 34:

To this day they do after the former manner: they don't fear YHVH, neither do they after their statutes, or after their ordinances, or after the Torah or after the mitzvah which YHVH commanded the children of Ya`akov, whom he named Yisra'el

What does this make you think of???

2 Corinthians 6:16
What agreement can there be between the temple of Elohim and idols? For we are the temple of the living Elohim —as Him said, "I will house myself in them, and I will walk among you. I will be their Elohim, and they will be my people."

Having other gods before YHVH isn't just a matter of worshipping specific gods... but putting anything before YHVH. Even serving YHVH can become an idol if it become more important than our relationship with Him.

Ephesians 5:5
For of this you can be sure: every sexually immoral, impure or greedy person—that is, every idol-worshipper—has no share in the Kingdom of the Messiah and of God.

Here Rav Sha'ul says that greed is the same thing as idol worship.

Philippians 3:19
They are headed for destruction! Their god is the belly; they are proud of what they ought to be ashamed of, since they are concerned about the things of the world.

Shalom v'brakhot v'simcha,
Moreh Chizkiyah Shlomo (Carl)