Answer

Hello A.,
First, you need to sharpen the question. What does "first and foremost" mean? What exactly are the two alternatives you’re torn between, and what practical difference follows from them?
 
It can be phrased in an extreme way: is there really such a thing as a Jewish nation, or is it only a religion?
Or perhaps: is the only essential characteristic of the Jewish nation its religion?
And in a less extreme form: is it possible to belong to the Jewish nation without the Jewish religion, or vice versa?
The question can be formulated in several other ways, and you can see that they differ from one another, even if there is some overlap between them.
 
At the next stage, after you formulate the question that interests you, you need to define terms. First and foremost, one has to define what a nation is. As is well known, there is no clear definition of that. It is a group of people with shared characteristics and a shared collective consciousness. In that sense, it is clear that there is a Jewish nation. Are Jews really any different from Belgians or Italians? By the same token, it is clear that this nation is not defined only on the basis of religion, since it includes quite a few people who are not religious. You surely know that the Archbishop of Paris, Lustiger, defined himself as a Jew of the Christian religion.
You can of course argue that these are Jews who are not behaving properly, but as long as you regard them as Jews, then we are dealing with a dispute between positions, not with definitions of the concept "Jew."
This of course leads to another question: who is supposed to determine the answer to your question—for example, whether there is or is not a Jewish nation, or what defines it? After all, that depends on whom you ask. Are you interested in sources? In your own personal position? In the position of the majority of the public?
I personally think there is a Jewish nation, and it is hard to deny that. In the amorphous sense in which every other nation is defined, such as the Belgians or the Italians, Judaism can be defined by some sort of collective consciousness, even without any connection to religion. But that is not an important question. What practical difference does it make whether there is or is not a Jewish nation? In substantive terms, your belonging to the Jewish nation or the Belgian nation is a neutral descriptive matter. It has no value significance. You can change nationality whenever you like and nobody will have any claim against you.
Even on the merits, every characteristic of a secular Jew can also be found in a non-Jew who, by everyone’s account, is not Jewish. There are quite a few non-Jews who speak Hebrew, pay taxes in Israel, serve in the army, read Hebrew literature, and take part in culture in Israel. Alternatively, there are quite a few non-Jews in the world who study the Hebrew Bible, and some even study the Talmud. There are quite a few Jews who do not speak Hebrew and do not pay taxes and do not serve in the army. So in what sense are they Jewish according to the national definition? Seemingly, only religion can define that. But a large portion of them are not religious either. What remains is a vague collective consciousness that is hard to define, and there is no real point in dealing with it. Whoever feels Jewish—good for him.
The question of Judaism has value significance only if we are dealing with the religious sense of Judaism. Belonging to the Jewish religion binds you to a set of values and norms. Therefore only the religious question has value significance. Everything else is not only uninteresting questions—there is really no question there at all. It is mainly semantics (depending on how you define nationality and whom you ask). But belonging to the Jewish religion is certainly a matter of religious and halakhic commitment. That is the meaning of Jewish religiosity.
I define a Jew as someone who is obligated to observe the commandments of the Jewish religion (even if he himself does not feel that way). That is a definition based on religion, but it includes people who are not religious. Who are those people? Someone born to a Jewish mother or someone who converted according to Jewish law. Of course, there is another question: who is an Israeli? Here the answer is different, and in my opinion there is no reason at all to tie it to the previous question. In Israel, because of the mixing of religion and state (it is defined as a Jewish state in some sense), people get confused about this.
See my article here:
https://mikyab.net/%D7%9B%D7%AA%D7%91%D7%99%D7%9D/%D7%9E%D7%90%D7%9E%D7%A8%D7%99%D7%9D/%D7%A2%D7%9C-%D7%96%D7%94%D7%95%D7%AA-%D7%99%D7%94%D7%95%D7%93%D7%99%D7%AA-%D7%91%D7%96%D7%9E%D7%A0%D7%A0%D7%95-%D7%95%D7%91%D7%9B%D7%9C%D7%9C
And also in Column 130. Also in Columns 336–339.
And more generally in Column 251. There I discuss the debate about definitions.
So there you have, in brief, an example of a question over which much ink has been spilled, with different opinions and major, heated arguments, when in fact it is just semantics and not worth dealing with. There are quite a few other questions like that.