Torah Sources

For the Jew, Christianity (the belief in Jesus as lord and savior) is an irrefutable form of idolatry.
The prohibition against and rejection of idolatry is at the very core of Judaism.

The following is an excerpt from Rabbi Aryeh Kaplan's book,
The Real Messiah? a Jewish Response to Missionaries

What can a Jew lose by embracing Christianity?
The answer is Everything.

"Christianity negates the fundamentals of Jewish faith, and one who accepts it rejects the very essence of Judaism. Even if he continues to keep all the rituals, it is the same as if he abandoned Judaism completely. The Talmud teaches us, “Whoever accepts idolatry, denies the entire Torah.” A Jew who accepts Christianity might call himself a “Jewish Christian,” but he is no longer a Jew. He can no longer even be counted as part of a Jewish congregation.

Conversion to another faith is an act of religious treason. It is one of the worst possible sins that a Jew can commit. Along with murder and incest, it is one of the three cardinal sins which may not be violated even under pain of death."
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Below is just a sampling of the many sources found in Tanach which proclaim the Oneness of Hashem, and warn against Idolatry

TORAH SOURCES:
Shemot (Exodus) 20:2-3
"I am the L-rd thy God, who brought thee out of the land of Egypt, out of the house of bondage. Thou shalt have no other gods before Me" ("You shall not recognize the gods of others in My presense").

Shemot (Exodus) 20:18
"One who brings offerings to the gods shall be destroyed - only to Hashem alone!"

Bamidbar (Numbers) 23:19
“G-d is not a man that He should lie, nor a son of man, that He should relent.”

Devarim (Deuteronomy) 4:15
“Take good heed of yourselves, for you saw no manner of form on that day when G-d spoke to you at Horeb...”

Devarim (Deuteronomy) 4:35
“Unto you it was shown, that you might know, that the L-rd is G-d, there is none else besides Him.”

Devarim (Deuteronomy) 4:39
“Know this day, and lay it in your heart, that the L-rd is G-d, in the heavens above and on the earth below, there is none else.”

Devarim (Deuteronomy) 6:4
"Hear, O Israel: The L-rd is our G-d, the L-rd is One".

Devarim (Deuteronomy) 11:16-17
“Be careful lest your heart is seduced, that you turn away and serve other gods and that you bow down to them. And then the L-rd’s anger will burn against you and He will shut up the heavens and there will be no rain and the land will not yield its produce. You will be quickly cut off from the good land that the L-rd is giving to you.”

Devarim (Deuteronomy) 13:1-2
"Everything I command you that you shall be careful to do it. You shall neither add to it, nor subtract from it. If there arise among you a prophet, or a dreamer, and he gives you a sign or a miracle. And the sign or miracle comes to pass, and he calls on you, saying, ‘Let us go after other gods, whom you have not known, and let us worship them.’ You shall not listen to that prophet or dreamer. For G-d is testing you, to see whether you love the L-rd your G-d with all your heart and with all your soul"

THE PROPHETS
Yirmiyahu (Jeremiah) 16:19-20
"To You the nations will come from the distant parts of the land" and they will confess, 'Our ancestors inherited falsehood, futility that has no purpose. Can a man make a god? The man not being a god?'"

Yeshayahu (Isaiah) 40:18
"To whom can you liken God? Or what likeness can you compare to him?"

Yeshayahu (Isaiah) 42:8
"I am G-d; that is My Name. I will not grant My honor to anyone else or My glory to idols."

Yeshayahu (Isaiah) 43:10
"You are My witnesses," says G-d, "and My servant whom I chose," in order that you know and believe Me, and understand that I am He; before Me no god was formed and after Me none shall be."

Yeshayahu (Isaiah) 43:11
" I am G-d, I alone. There is no Savior besides Me."

Yeshayahu (Isaiah) 45:5
"Thus says G-d, the King of Israel and its Redeemer, the L-rd of Hosts, "I am the first and I am the last, and there is no god besides Me."

Yeshayahu (Isaiah) 45:22
"Turn to Me and be saved, all the ends of the earth, for I am G-d, and there is no other."

Yeshayahu (Isaiah) 46:5
"To whom can you liken Me and whom can you declare My equal? To what can you compare Me that we should seem similar?"

THE SIN OF IDOLATRY
The following is an excerpt of a commentary by Rav Mosheh Lichtenstein on the Haftara reading in Yeshayahu and Yirmiyahu for Parashat Shemot. Read it in full at the Virtual Beit Midrash site.

"....other sins are forbidden and negative acts, but they are performed in the framework of a relationship between Israel and God. He is our God and we are His people; the sins do not cancel the connection between man and God, but rather they are committed within its framework. Idolatry, on the other hand, cancels the very connection, because it indicates preference for the idol over God as master, father and spouse. It is for this reason that Scripture is full of metaphors for fidelity, betrayal, and promiscuity regarding idol worship. This is because it is the question of basic and fundamental fidelity, and not the fulfillment of specific particulars, that stands at the heart of the matter. It is only with respect to idolatry that the Torah uses the term kin'a, "zealotry" (see Ramban, Shemot 20:3), because it has pretensions of substituting for God and creating an alternative relationship with Israel. In light of this, Yeshayahu focuses the exile on idol worship, for only that can lead to Israel's expulsion from its "house," i.e., Eretz Israel. It is because of idol worship that the people of Israel are regarded as renegades who are unfit to live in God's place...

...The sin of idol worship is not necessarily committed out of the pursuit of pleasure and the setting of man in the center. On the contrary, it often appears in periods of spiritual tension and religious seeking. The sinner does not loath spiritual life and the desire to conjoin with a being that is beyond his world, but rather he errs in his choice. His sin is a sin of treachery."