You make a good point here. Let me clarify that I meant by denominations- not really by individuals. There are individuals who say they are Christians, but in reality, they are just paying lip service to the entire thing and not putting it into practice- and it can be very apparent when they do that.
However, I don't feel qualified to tell people are different denominations that they are NOT true Christians because I believe in for example, full body immersion baptism when other denominations only do sprinkling. It is a bit tougher when it comes to believing in the Trinity- as a Baptist, it is one of the teachings, but I know of certain individuals, and denominations that reject the trinity- I believe that you do, Shermana (you can correct me if I'm wrong). And there is a difference of opinion of whether Jesus rose physically during His resurrection or only spiritually. I, personally, believe that Jesus rose physically, but I am not going to point and say "You are not a true Christian" to someone because that person believes He was only raised spiritually.
Also, keep in mind that everything I say is only my opinion and any opinion can be wrong and disagreed with.
I most certainly do reject the Trinity. In fact, I'd even use that as an argument that I'd say those who think Jesus was God generally don't have a remotely close understanding to what the Messianic Prophecies were about in the first place, and what it means to be "Christ" to begin with. I could argue that belief in the Trinity demonstrates a willingness to side with the historic majority in the face of scholarly and reasoned counter-argument and interpretations. One can ask: "Do they truly believe that the text unequivocally says that Jesus was God and that the Nicene arguments for the Trinity are true because they agree with the scholarly and grammatical concepts or is it because they want to fit in with the majority"? A great example, what was in the hearts of those who went by the newly invented "Colwell's rule" in the 1930s? Were they honestly believing that they made some groundbreaking grammatical discovery or were they trying to help prevent a scholarly argument that would pose a threat to the status quo to the point that they'd be willfully dishonest about Greek grammar? What can be said about the hearts of those who believe Colwell's rule and ignore or reject all the arguments against it? Is their thought well reasoned? Have they researched it? Do they understand it? Or do they just repeat it because of where their hearts lie on the matter? Same can be said for every argument of the Trinity. Do they believe in it because they really think its true based on all the evidence or is it because their heart is set on fitting in with the rest?
How many Christians do you think would be prepared to defend why they think Yeshu Ben Yusuf was the Messiah and why they so earnestly accept this claimant as lord other than the popularity factor?
But I think differences in details are a different category, but it can still relate to how one can see one's character. If one believes Infant baptism is sufficient, that can possibly show that A) They don't really think much of baptism being of value regarding one's conscious understanding of what they are doing and that it's just a ritual that causes some effect of sorts (assuming they can argue what that effect is) and B) They may be going by a traditional doctrine guiding their opinion as opposed to what the text may indicate on what Baptism was intended to be and what it does.
On the other hand, the question of whether Jesus rose physically or alone in the Spirit I don't think has much bearing on the actual teachings and lifestyle choices of a believer. That's more of a matter of interpretation that doesn't involve the spectrum of behavior and action. However, it can be said that those who believe Jesus rising from the dead somehow means all their sins forever are forgiven and that their works don't matter are demonstrating where their hearts lie in that regard. As opposed to the issue of obeying the commandments and what Jesus taught which doesn't have much room for debate.
My main issue however is the teachings themselves as well as the basis for what Jesus was and was teaching, which was the Jewish context. But in this instance, if a person thinks baptism isn't necessary at all, when the text says that the disciples ARE to baptize, what does that say about their hearts? That they are willing to ignore Jesus's command for the sake of a theology they find less burdensome?
Trust me I'm aware that everything anyone says is everyone's opinion. Including mine! But I'm nonetheless using such statements to show where my own opinion stands in relation.