Saying something is "for real" vs just saying something is "real"

Solution 1:

For real indeed has a kind of slangy/childish connotation, which I'm sure is why it was used in the title of that movie (a quick Google search reveals that the film is about a four-year-old who has a near-death experience).

However, it also does not mean the same thing as real: if something is real, it exists, while if something is for real, it is legitimate.

Merriam-Webster backs up this adjectival sense of for real (definition 3.2 and 3.3 at that link):

2: genuine "couldn't believe the threats were for real"

3: genuinely good or capable of success "not yet sure if this team is for real"

To be fair, that dictionary also gives genuine as one definition of just plain "real", but I do think that "for real" is much more about legitimacy than "real", to the point that there is a definite difference in meaning. Consider the following examples (mine, this time):

A: I don't think this pizza is real.

B: I don't think this pizza is for real.

If I were looking at a plastic replica of a pizza, I could say A but not B. In contrast, if I had just been given a pizza that I was told was from a famous restaurant, but in fact looked very unappetizing, I could say B but not A.

Of course I contrived that example to demonstrate the difference in meaning. To return to your original question, Heaven is real and Heaven is for real do indeed mean pretty much the same thing. But my point was to show that real and for real are not always interchangeable.

As for the grammar part of your question, I don't think for real is grammatically remarkable - there are other structures of the form "[Noun] is for [adjective]", e.g. This coffee is for free.

Solution 2:

It is definitely slang. But I believe it is more synonymous with Serious, or Seriously... "Is he for real?" or Person A: "I just jumped over that garbage can" Person B:"For real?" That is the best explanation I have for you. But it is simple, if you are using this specific slang you are probably in a very informal setting.

Solution 3:

Here, for seems to be being used as a kind of intensifier, a marker to increase the rhetorical weight of the word real.

In other words, it does not merely state the purported fact of the realness/reality of Heaven, it positively tries to assert it.

As far as I can see, it doesn't have any genuinely grammatical function when used in this way.