if you look at the teachings of Jesus Christ from the three gospels, Mark, Luke and Matthew, it would seem that there is a fundamental problem with "just believing"
the whole point of christianity (I can't speak for other religions, i've only rejected one religion, and that experience was enough religion for me, thanks) was to be a good person, that's what many christians claim when they say that atheists can not possibly be moral people, because atheists are non-believers.
the central message of Christianity is "do unto others as you would have them do unto you" ... aka "love thy neighbour as you would have them love you"... aka. "the golden rule" .. this is the second commandment that JC taught... the first being "love thy god before all others" ..
at this point, it's pertinent to mention that these commandments are actions that you must *do* ... unlike some of the ten commandments where you can do nothing and still manage to follow them perfectly... ie. "thou shalt not kill", I'm following that one right now, and i suspect that at least six billion other people are doing the same right now as well.. but just because we are all following that commandment, that doesn't make us good people.
the point being is that if you "just believe", then how does that make you a good person when your messiah taught that you must *do* good things in order to be this good person?
this is a fundamental problem if you are claiming to follow JC, as it would seem that "just believing" in Jesus is idolatry: as that turns "Jesus" into a thing or an icon that equates to being a good person..
and in spite of what people have read from the book of John (esp John 3:16) the other three gospels state that you must do good things in order to be a good person in order to gain salvayshunn in da heavenuh, so thinking that "just believing" in JC makes you a good person, goes against the teachings of Jesus. ie. JC taught that you must take actions to be a good person, "just believing" is not taking an action so it is not what JC taught.. eg. the three gospels of Mark, Luke and Matthew, beats the one gospel of John, right?
so if you think that you are a good person by "just believing" in Jesus, then you are not believing in the Jesus from the bible, you believe in a *false* Jesus that you've made up.
This Jesus of yours may be similar to the "real" Jesus from the bible, but it's not, because you just can not ignore the two commandments that Jesus taught and claim to be a christian.. so if you "just believe" in this Jesus version of yours that supposedly said to "just believe" then you are worshipping a false Jesus, which is a false god and thus breaks the first commandment ...
just because the image you have of Jesus is merely in your mind, it's still does not negate the fact that you are committing idolatry, in fact, the easiest place to commit idolatry is right there in your mind; ironically the non-believer is inherently not capable of idolatry, so when they die, they could therefore end up in heaven *accidentally* if they are not careful
the USA, with its having "god" on currency and in the pledge is a massive example of idolatry on a national scale, it turns god into some icon that you see all the time, and i have no doubt that would lead to more of that "just" believing stuff where you simply think you are a good person because it says "in god we trust" on money, or that you pledged allegiance to god the flag and country or whatever for the ten years you went to school
"just believing" in JC would seem to be a massive problem if you think your eternal life is dependant upon it, especially in the USA; and even if the other person is actually kind and follows the teachings of Jesus to the letter, if they say to you that you should "just believe", then they are asking you to commit idolatry, because you can't "just believe" unless you go and do good things in order to be saved, or so the story goes