Most practitioners of Vodun and diaspora systems are Christian, so I would say this is just a North American and culturally lost or disgruntled African thing. Mainly, it’s because they simply do not know what an Orisha is. For an educated or non eurocentric Christian, to call an Orisha a demon is like calling water or air a demon. So please don’t lump all Christians into the same category. Someone wearing a cross or going to church does not mean they are ignorant. In fact, there are many Christian churches that explicitly embrace an African style of worship.
The problem among many Black people is that Christians called to ministry tend to be Obatala oriented people. Without the proper guidance, they tend to be overly dogmatic, and will divide Nature into good and evil in small, limited, human terms instead of universal, natural terms. I saw this play out in my own family, and though it caused me much pain in my childhood, understanding them in Vodun terms helped me to understand and forgive them.
So I hope that as I explain this to those of you who have asked this question, you will also understand and forgive the Christians who have broken away from their ancestral faiths.
Having only one god is easier than having many. In Judaism, they call the one God by a plural name because He is all-encompassing. They sought to remove the illusion of division among the Spirits because this is ideal. It is however, too big a stretch for the vast majority of human beings. The vast majority will, due to their psychological limitations, rather than have a God who is Eshu-Olodumare-Obatala-Shango-Yemaya-etc, will under these circumstances, choose one and raise him/her above the others. For some Christians, this is basically a war god who, like a god, embraces and blesses those who believe in him and follow his commandments, but like a human, shuns and damns those who do not believe in him or follow his commandments, which are filtered not through Nature, but through the minds and wills of humans.
The Jewish God who commands Jewish people to force themselves to remove the division and still view Him-Her (Her&Him is, according to some scholars, the true and literal meaning of the word Yahweh) as all-encompassing, also commanded the Jewish people not to mistreat or look down on others. Their leaders from Moses on, felt it would be best for them to separate themselves from others in order to achieve this, but all people are supposed to be considered by them sons of Noah. So long as a person lives a righteous life, it does not matter if their pantheon is one or one billion, they are, for all intents and purposes, saved.
Monotheism was not to be forced upon people of other nations. Monotheism was for sons of Abraham who, having a divine revelation, wanted to make a people who did not have the divisions that others did. Just as the idea of monotheism has its vulnerabilities that can be exploited in the human psyche, so does polytheism. A state religion could arise that twists thoughts about the Spirits just as a state religion can twist thoughts about the Holy Spirit.
The sons of Abraham were to make a state that was dependent on the All-encompassing God, not a king. The priesthood was not invented until Moses, and even then they were under strict limitations. It was about what sons of Abraham were supposed to do and believe, not enforcing their beliefs on the world. The world, as everyone can plainly see, is not capable of functioning without human leaders. Ultimately, the Jews were not either, so they chose Saul, and it was downhill from there.
Eventually, Elijah had to do something that I am sure would make Abraham weep, and fight what had become the state religion in Israel, the land nation with God as the only king.
Now you have the context for what has become Christianity in too many situations. When Jesus came, he wanted to get the Jewish people back to that ideal of living in the Spirit, not just the law. He never attempted to convert any of the Pagans he helped, to Judaism, or get them to worship him, even though this would not be wrong. Jesus is a prime candidate for an avatar of Obatala. If any man who ever lived on Earth could be said to be an emissary of the Almighty, it’s him. So I have no problem whatsoever with people who focus on Jesus, especially when their cultures have been ravaged to the point that they no longer have the social context for other faiths.
The problems start when people are incapable of following the path that Jesus did, and wrap their brains around the idea of a One God. So instead of embracing the ideals of Judaism, as Jesus did, they put limits on who God is. Like the Jews lost their way in Biblical times when they chose to have a man as king instead of God, too many Christians today make God a man…a petty man who is somehow reduced or insulted by the equivalent of someone saying, “Wow, God, water is wonderful! I want to make an offering to say how wonderful water is. Water is where all physical life on Earth came from, and I want to make sure I am showing it the proper respect. I want to call water Great-water and give her flowers…”
The idea of Yemaya takes nothing away from Obatala or the Unfathomable Omnipotent Be-ing he represents. However, if a person has been taught that they must have only one god, and they are incapable of or unwilling to widen their view, Yemaya is a competitor.
Though the Orishas are not competing with God, and have in fact been assigned by God to specific purposes, and even in the true monotheistic sense are, at worst, a face or aspect of God, some Christians view them as competition. Though like Muslims who do the same, they are dangerous, it is important for us to understand that they truly “know not what they do”.
For this reason, I take a sort of hard stance, and don’t get too embroiled in arguments with them. Spiritually, it is like arguing with a 2 year old who refuses to see the world beyond their little bubble they wish was reality. On the other hand, as annoying as it is, if it is keeping them sane and happy, I can’t knock it. Where I draw the line is when it makes them insane, unhappy, and leads them to harm others.
If you find yourself in a situation of being vulnerable to or dependent on people who think you’re worshipping demons, treat them like kids who believe in Santa Claus, just with more power to harm you. Do what you need to do to survive, and don’t try to wake up a sleepwalker.