I'm not really sure what you mean by "dative" in English, as there isn't really an accusative/dative distinction - in situations where other languages might use a dative, either the accusative is used ("I gave him the book") or a preposition ("I gave the book to him"). However, the following might be helpful in articulating why "who" can be used, and may even sound better, where some insist on "whom" - whereas in other situations "whom" is still preferable:
To make my explanation clearer (at the expense of much precision, for which please forgive me) I'll refer to two "styles" of English - one very formal (in which the who/whom/whom prescribed by the style guides is compulsory), and one much more colloquial (in which who/who/who rules the roost, and "whom" is seldom if ever used). Very loosely speaking these correspond to English as it was both spoken and written in the past, and how it is most often spoken today; since trends in the written form often follow those in the spoken we might see current written English as being in a transition between the two.
Given all that, the sentence:
The man whom I saw yesterday was tall.
entirely follows the rules of the formal style, and is thus acceptable.
The man who I saw yesterday was tall.
entirely follows the rules of the more colloquial style, and is thus acceptable
However, the sentence
*The man to who I gave the ball yesterday was tall.
grates. This seems to be because it follows neither the rules of the formal style (which would have "to whom"), nor the colloquial style (which would instead have "The man I gave the ball to yesterday was tall", or a variant thereof), and is thus unacceptable in either. Similarly,
*The man whom I gave the ball to yesterday was tall.
also falls between both stools.
Note that this is largely handwaving, rather than a rigorous argument, but it's interesting to note that studies have been performed in cultures exhibiting diglossia (i.e. using "high" and "low" variant forms of what by some definitions could be considered one language, in different contexts) where subjects were shown words or sentences combining features of the "low" and "high" variants. It was found that some of the features were only weakly associated with one variant or another, in the sense that (say) using one from the low variant in a sentence otherwise fully "high" would not render it unacceptable; however other features were "strong" in the sense that a sentence containing features strongly associated with "high" and others with "low" would definitely render the sentence unacceptable. It is possible that, on a much smaller scale, a similar phenomenon is going on here (though of course it would be very bold to assert that this is the case without much more rigorous research!)
The Wikipedia link about the accusative case explains that
Modern English, which almost entirely lacks declension in its nouns,
does not have an explicitly marked accusative case even in the
pronouns. Such forms as whom, them, and her derive rather from the old
Germanic dative forms, of which the -m and -r endings are
characteristic.
Now, whether to use who or whom in your sentence entirely depends on which case should be used, accusative (whom) or nominative (who).
In English, it is grammatically correct to use nominative after the verb to be as in
It's he who stole my car. It's they who told me the truth. It's she
who lied to me.
However, we know that "It's me" (using the accusative case after to be) is broadly used in English. But it is just a few exceptions.
In your sentence, it is appropriate to use the nominative case as it is the complement of to be. If you divide the sentence into two parts:
He must decide / He should be who => He must ask who he should be => He must decide who to be.
in the same way as:
He must decide / He should meet whom => He must decide whom he should meet => He must decide whom to meet.
He must ask / She is who => He must ask who she is. (This question cannot be shortened with wh-word + to-infinitive as the subjects are not same.)
We don't ask,
*Whom is he? or *Who is him?
*Whom am I? or *Who am me?
because whom and him/me are the accusative case and can't be a complement of the verb be in this case.
Note: "He must decide who he wants to be" is more idiomatic than "he must decide who to be".
Best Answer
If I could choose neither, I would, since I'm not sure if the sentence is grammatical (I have asked a separate question about that here: Is "I am who(m) God made me" grammatical?).
If I had to choose one and I was allowed to choose based on my own preferences, I would choose who since, as many posts on this site explain, "whom" often sounds stuffy or pedantic.
But if I had to choose one based on what I think fits best with prescriptive grammar, I would choose whom.
This isn't a complete answer, so much as a summary of what I've discovered while researching this question. I hope some syntax expert will post a more a definitive answer.
Syntax: the "object complement"
In sentences of the form "[God] [made] [me] [a man]," "God" is the subject, "made" is the verb, and "me" is the object of the verb. The phrase "a man," rather than being an object, serves the role of what is called an "object complement."
The grammatical case of the object complement
This is tricky to ascertain, since most sentences with a pronoun in this position sound very awkward and unnatural ("God made me her"?/"God made me she"?).
However, I think there are several fairly strong arguments for it being in the objective case.
For one thing, I was able to find one fairly common expression that uses this structure with an objective-case pronoun as the object complement: "what makes me me" and variations on this phrase. As far as I can tell, no one say "what makes me I," and my intuition tells me that this would be ungrammatical.
Unfortunately, this is not a foolproof argument because online examples that are similar to this often show some odd punctuation that casts doubt on the role of the pronoun here. Often there is a comma or ellipsis before the pronoun: "What made me, me", "everything that made me ... me". In contrast, a comma would be ungrammatical in "God made me, a man." This might be a sign that these phrases have different grammatical structures.
Sometimes, what appears to be the object pronoun is capitalized or put in quotation marks: "What events in my life had made me, Me?", "the things that made me 'me' ". This odd treatment almost suggests the word "me" in this expression is being treated as a noun rather than a pronoun, and in that case it would not inflect for case and would be useless as evidence for the "who"/"whom" rule. (A parallel case: we say "The Me I Want to Be", not "The I I want to Be," but prescriptive grammar still prescribes the nominative form in the phrase "who I want to be.")
But, I think there are also theoretical arguments that suggest the object complement is in the objective case (or at least, that it "should" be in prescriptivist grammar). The most straightforward is that it's a complement, and prescriptivists often value arguments like "complements should match the case of their referent." That's the whole foundation of the argument for saying "It is I" rather than "It is me."
From a more descriptive viewpoint, I have read that the "objective" case or pronouns is generally less marked in modern English, which I think means that phrases tend to default to that unless explicitly assigned to the nominative case by a rule. However, the objective case of "whom" is an exception since it basically only occurs in educated constructions that have to be explicitly taught. Some linguists argue that the rules for using "whom" have actually become distinct in the modern language from the rest of the English case system. (Some papers I found about this that I still have to finish going through: The Who/Whom Puzzle: On The Preservation Of An Archaic Feature, Whom and the English Case System).
Is it grammatical to replace the object complement with a pronoun?
I'm not sure if it's grammatical to replace this element of the sentence with a fronted relative pronoun. (I believe "who" or "whom" in this context would be a relative pronoun, although I'm not sure: it's possible it would be an interrogative). I'm currently trying to research this.
There are some examples of sentences with object complements here: http://www.englishgrammar.org/object-complement/
I don't get great results when I try to modify them to use the pronoun "what":
These all seem awkward to me, although Mitch and Araucaria have left comments indicating that "What they named the boy was Christopher" seems OK to them. The comments that used to exist below this question indicate that many people think "I am who(m) G-d made me" sounds ungrammatical, although I have not found any source that explains why it would be.
Araucaria also found some real-life examples of "I am/ This is who God made me" that I think are worth listing in an answer:
According to typical prescriptive rules, the pronoun's role in the matrix clause shouldn't affect its form
The matrix clause (or main clause) in this example is "I am __." In general, the choice of "who" or "whom" is not affected by the pronoun's role in the matrix clause, only by its role in the embedded clause. So the prescriptivist rule about using the nominative form of a pronoun after "I am" should be irrelevant. (A similar example is discussed in this following question: Who vs. whom in complex sentences—"I gave the prize to whoever deserved it most" is correct, even though we would say "I gave the prize to him/her," because the case is determined by the embedded clause "deserved it most" and we we would say "He/She deserved it most.") The following questions are also relevant: Can a phrase be the object of a clause and how would its subject change?, "I give it to him who came first" vs. "to he who came first", Which is grammatically correct: "Let he who..." or "Let him who...".
For some speakers, there are additional complications in phrases such as "the person whom the police thought was responsible," but as Geoffrey K. Pullum explains in the linked article, the use of whom in this context is generally considered incorrect by prescriptivists.
A caveat: F.E.'s answer to the following question is also relevant, and points out that in real English usage (as opposed to the rules of prescriptivists), "fused relative" constructions like this with clashing case requirements may be avoided: "Put me in touch with whomever created it"?
Wikipedia cites an interesting passage from "The Distribution of Pronoun Case Forms in English", by Heidi Quinn, that indicates that in a similar construction in Old English (the "argument relative"), the case of the wh-pronoun was based on its role in the matrix clause.
(p. 331)
Unfortunately, the page with the relevant sentence was not visible in the Google Books, but I found what seems to be the same example in a PDF of Quinn's thesis:
I'm not actually sure based on this example if this construction is really analogous to the modern English construction, but it's interesting nevertheless. You can see more discussion on the Wikipedia talk page.
Quinn also mentions the typically limited distribution of who(m) in free relatives that Peter Shor mentioned in his answer:
(p. 333)