You should incline your prayers, your thoughts and your actions only to G-d. This is the fifth principle and it warns against idolatry (עבודה זרה / avodah zarah -- literally "strange worship"). Do not pray to or through any intermediary. Do not pray to an angel. Do not pray through a human (e.g. a deceased relative). Do not pray to or through anyone or anything but G-d Himself.
It is inappropriate to pray to anyone but G-d Himself.
Worship of any three-part god by a Jew is nothing less than a form of idolatry. (Whether Christianity is idolatry for a non-Jew is debatable). The Rambam certainly saw it as idolatry, but many others opine that for a non-Jew Christianity is שִׁתּוּף / shituf. Shituf is the heretical idea of some sort of co-mingling of something with G-d -- the worship or belief of other gods (e.g. Jesus and the holy ghost) in addition to the G-d Himself In the case of Christianity this is the idea of a G-d in human form being worshiped (Jesus) and even the “holy spirit” being worshiped.
Whether Christianity is שִׁתּוּף / Shituf or עבודה זרה /avodah zarah for a non-Jew, it is definitely עבודה זרה / avodah zarah / idolatry for a Jew -- and forbidden. Some posit that today’s non-Jewish Christians cannot be considered idol-worshipers since they are merely following in the tradition of their parents. Still, most agree that Christianity is idolatry for Jew and non-Jew since it does involve praying to or through an intermediary (Jesus, and in the case of some Christians praying to or through saints, Mary, etc.).
Nissim ben Reuven, The Ran, (14th century) wrote “...even the Christian saints, and even the...leader of the Ishmaelites, even though their followers do not consider them gods, nevertheless, since they bow to them to acknowledge that they are human incarnation of their divinities, they all have the halachic status of avodah zarah...”
Idolatry does not necessarily mean worshiping a god of stone or wood. Even if a Jew worships the highest angel, it is also a form of idolatry. G-d is the infinite One, Creator of all things. Anyone who worships anything else is guilty of idolatry even for a non-Jew.
We have already discussed the unity of G-d. The three-part god of Christianity is not the G-d of Judaism.
Therefore, in the Jewish view, Christianity may very well be a variation of idolatry even for a non-Jew. The Christian bible teaches its followers that the only way to G-d is through the son (Jesus). John 14:6 “Jesus answered, “I am the way and the truth and the life. No one comes to the Father except through me.”
This is in direct violation of the Torah precept to worship only G-d. Sh’mot / Exodus 20:2-3: “I am G-d your L-rd, who brought you out of Egypt, from the place of slavery. 20:3 Do not have any other gods before Me.”
By saying "before Me" G-d is stressing that no one should believe in any other deity, even if you believe in Him, too. By praying to or through Jesus a Christian is putting that other entity between themselves and G-d. If you believe in G-d, why do you need Jesus? Why do you need to pray to or through anyone but the Holy One?
G-d gave man absolute free will. Man can choose good or evil, the blessing or the curse. The choice to honor G-d and pray only to Him is an expression of our free will. So that such a choice can exist, G-d created a world where both good and evil can freely operate. He thus said, "I form light and create darkness; I make peace and create evil; I am G-d. I do all these things" (Y'shayahu / Isaiah 45:7).
G-d created it in order that man should overcome it. It is written, "Behold, the fear of G-d, that is wisdom, and to depart from evil, that is understanding" (Iyov / Job 28:28).
So humans have absolute free will, with the ability to choose between good and evil. The Torah says: "I call heaven and earth to bear witness this day, for I have set before you life and death, the blessing and the curse. Therefore, choose life, so that you and our children may live" (D’varim / Deuteronomy 30:19).
It is evil and a curse to pray to false gods or intermediaries.
People, upon hearing the word “idolatry” picture people bowing and praying to statues. This is not the Jewish definition of idolatry. The term for idolatry in Hebrew is עבודה זרה / avodah zarah / idolatry – and it translates to “strange worship.”
What about Jews who say prayers at the gravesides of famous Jews? There are Jews who visit the graves of our patriarchs and leave prayers. There are Jews who visit the grave of the Lubavitcher Rebbe, Menachem M. Schneerson. Would this not be considered שִׁתּוּף / Shituf or עבודה זרה /avodah zarah?
No, because Jews do not pray to the dead. When a Jew visits the gravesite of a sage, they are not praying to or through that person. They are not asking the dead person to "intercede" on their behalf. 128:13 in the Kitzur Shulchan Aruch says "the place of the burial of righteous people is holy and pure, and prayer is accepted there more [readily], since the person is on holy ground. And the blessed holy one will do kindness in the merit of the righteous people. However, the person parying should not face the graves (literally: corpses who dwell there) as they pray, because the matter is close to being in the category of "you shall not. . . communicate with the dead." D'varim / Deuteronomy 18:10-11. Rather, he should request from the blessed G-d that He have mercy on him in the merit of the righteous people, the dwellers of the dust."
G-d describes avodah zarah (idolatry -- literally "strange worship") in D’varim / Deuteronomy 13:7-9 – and it is the death knell to Christianity – even to those who do not “worship” Jesus but claim to only “pray in his name.” -- it is still a clear violation of the Rambam’s 5th principle against worshiping any other than G-d.
Even Christians who say Jesus is G-d are violating the precept since no one at Sinai knew or prayed to or through Jesus. The Torah says: “[This is what you must do] if your blood brother, your son, your daughter, your bosom wife, or your closest friend secretly tries to act as a missionary among you, and says, 'If your brother, the son of your mother, tempts you in secret or your son, or your daughter, or the wife of your embrace, or your friend, who is as your own soul saying, "Let us go and worship other gods, which neither you, nor your forefathers have known."' 13:8 [He may be enticing you with] the gods of the nations around you, far or near, or those that are found at one end of the world or another. 13:9 Do not agree with him, and do not listen to him." D’varim / Deuteronomy 13:7-9.
The key phrase here is "WHICH NEITHER YOU OR YOUR FOREFATHERS HAVE KNOWN."
Jesus supposedly lived 2000 years ago, 1300 years after our fathers heard G-d speak at Mount Sinai. Thus the worship of Jesus is an experience that we did not know (at Mount Sinai when we accepted the Torah) -- and thus is worhsiping Jesus (or praying "through" him is idolatry for a Jew.
Line 19 of that same passage (D'varim / Deuteronomy 13) tells us "For you shall hearken to the voice of the L-rd your G-d, to keep all His mitzvot which I command you this day, to do that which is proper in the eyes of the L-rd, your G-d."
Those mitzvot are the same ones NOT observed by Christians. Christians (non-Jews) are only bound to 7 mitzvot, but one of those is to worship only G-d. While some Christians view Jesus as a human being, most worship him as a part of G-d (shituf at a minimum for a non-Jew).
Praying to or “through” Jesus was not a spiritual experience known to our ancestors at Mount Sinai – hence it is forbidden to a Jew. Again, the Rambam was simply stating a fact already well established in the Torah.
When did "our fathers", present at Sinai, have a spiritual experience with Jesus? Was Jesus "known" to them? Did they pray to Jesus or through Jesus? Of course not! Jesus was unknown to them.
G-d warned us against both Christianity and Islam -- any spiritual experience not known to us at Mount Sinai is false. It is not the Rabbis who have "changed" or turned to avodah zarah -- it is those apostate Jews who pray to or through a man and put that man above the G-d of Abraham, Isaac and Jacob.
Principle number 5: G-d is the exclusive object of worship. No one or thing is worthy of being worshiped.