While there may be forms of Christianity which are less problematic than others, the underlying idea behind the religion is problematic. (Some forms reject the trinity and some forms don't practice the symbolic cannibalism others do) The fundamental nature of Christianity runs counter to what Judaism established.
- Their religion is based around a human sacrifice for sin atonement. That concept contradicts very fundamental ideas in Judaism. Judaism rejects entirely the idea that a human sacrifice should occur at all. Our worst enemies practiced human sacrifice. Our rejection of that is part of what separated us from other nations at the time. Hashem separated himself by rejecting a human sacrifice (Isaac) and requested a ram sacrifice instead. That entire narrative relates to how human blood for sin is not how Hashem operates. When you add Jesus into the equation (a being they claimed was God in flesh) and then state God sacrificed a person (who was himself at the time) for sin forgiveness, it contradicts everything we established in Torah. Why would Hashem break his own standard? That makes no sense.
- Their nature of sin itself is different. Judaism establishes that we each carry our own sins. We are not responsible for the sins of our families or ancestors unless we carry on their mistakes and continue them. Outside of that, we all have our own moral bank account. Christianity changed that relationship with the concept of Original Sin. They state that we all carry the universal burden of the sin Eve made when she disobeyed God (the original sin). While it's true her actions fundamentally changed the nature of humanity (and made us as we are) the fact is Judaism rejects the idea that Hashem has added a sin debt to every human by default because of her actions. Our sins are our own and her sins were hers. We don't all start off in the negative because of her actions. Hashem is just and such a standard would be the opposite of justice.
- Their fundamental belief in Christianity is that you need Jesus for heaven. That statement is the "no, no" to end all "no, nos". Jesus fundamentally changed the system by stating that no one can get to the father except through him. That statement is a gross violation of everything established beforehand and it created a situation where a middle-man positioned himself in a such a way where you needed him piggybacking onto your teshuvah process if you wanted full forgiveness. Nothing about that has anything to do with Judaism. It's a perversion of it to build something new around one man.
How about being a Noahide and rejecting Christianity as a doctrine,
but taking part in Christian religious activities such as sermons,
What if I cheat on my spouse physically but I promise to never do so emotionally? That's still being faithful, right? (sarcasm)
The relationship between God and his people is described like a marriage. You're faithful to God in the same way you are faithful to your spouse. When you commit to your spouse, you remove yourself from temptations or inappropriate relationships which border on unfaithfulness.
- You don't have secret friends.
- You don't get physical with other men/women.
- You don't speak flirtatiously with someone as you would your spouse.
There are lines drawn to establish that respect.
Whether you are a Jew or whether you are a Noahide, if you are still participating in religious activities developed by Christian tradition, you're cheating on Hashem. You're giving an aspect of your divine life to someone else and pretending you're not being unfaithful because your "heart is in the right place" while doing it.
That's not how it works. When you take on foreign worship practices, you are cheating on Hashem.
There's a reason we no longer pray on our knees (except Karaite Jews) - It's because we actively wanted to separate ourselves from other faith practices we may unintentionally emulate. Risking that means risking cheating on Hashem and breaking our covenant as a nation.