Maimonides says in the second chapter of Hilkhot 'aboda zara: "The essence of the prohibition of idol worshipping is not to serve any creature or creation, neither an angel, not a constellation or a star, nor any material element, or any entity created from them. And even if the person who is worshipping [anything or anyone but God] knows and admits that HASHEM is the only God, the act is still considered 'aboda zara." In other words, if a Jew, a religious and righteous Jew whose faith in God is unquestionable, prays to our "mother Rachel" (Rahel Imenu), even if his intentions are pure, and he or she firmly believes in one God, according to Maimonides, this person is performing an act of 'aboda zara.
We are allowed to pray to HASHEM and ask Him that in the merit (=zekhut) of our matriarch Rahel (or Abraham, Ytshaq, Ya'aqob, etc) He will help us, redeem us, etc. We do that all the time. But our prayers have to be directed always and only to HASHEM, our God.
Maimonides says that a person who worships anything or anyone but God, and yet believes in God, is repeating the mistake of Enosh. "this person will be serving a created being in the manner that Enosh and his generation worshipped at the beginning...." . The generation of Enosh believed in God, but they prayed to the sun and the moon as intermediaries between them and God. They believed that since the sun and the moon are "closer" to God, it would be appropriate to pray to them, and somehow the sun and the moon will deliver his message to God. In a similar way that when I express my request to my employer's secretary, I'm sure she will relay the message to my boss. A good Jew might think: "I will pray to Moshe Rabbenu, Eiyahu haNabi, Rabbi Meir, Rahel imenu, etc. and they will surely deliver my message to God.
Maimonides asserts that this thought is wrong. And he warns against this practice: "...the thoughts of your heart should not lead you astray to worship these [entities] and make them an intermediary (sarsur) between you and the Creator." Praying to an intermediary is considered an act of idolatry, regardless of my right beliefs and my good intentions.