Historians and Old Testament scholar here: I will try to answer your question as succinctly as possible in two paragraphs, and for the sake of simplicity I will assume that by "the Jews" you mean a group of people who worship the God YHWH:

A sect is first understood to be a faith community that has split off from a larger, established faith community because its doctrine differs from the latter. There are other aspects to it, but they are irrelevant to us here for the time being. Following this basic definition, Judaism has never been a sect, since it did not emerge from any other established faith community (unlike, for example, Christianity or Islam). Although "Judaism" of the Iron Age shares beliefs and gods with its neighboring religions, it is clearly distinguishable from them by its focus on YHW(H) (be it as a family and tribal god, later as the main god of a monolatry (the worship of a single god without denying the existence of other gods) and finally, at the latest from the 5th century B.C., as the only existing god in a monotheistic faith system).

The followers of YHW(H) were at no time small or smaller than the neighboring religions of the time, which becomes clear at the latest by the fact that the YHWH faith was state cult of two kingdoms (Israel and Juda) from approx. 1000 to 600 B.C., even Israel already goes down in the 8 century. The writings of the Old Testament now go back to people who felt belonging to this God YHWH and cover a period of origin of over 1000 years. The Old Testament can therefore - despite cultural and religious influences from outside - be attributed to "Judaism" with a clear conscience.

Mark S. Smith: The Early History of God. Yahweh and the Other Deitries in Ancient Israel, Cambridge, 2002.

Peter Mommer: Einführung in das Alte Testament, Berlin 2015.

Walter Dietrich (ed.): Die Welt der Hebräischen Bibel. Umfeld - Inhalte - Grundthemen, Stuttgart 2017.