The oldest monotheistic religion still in existence today is Judaism, from Abraham onward. For Jews, that is a religious truth and is stated by Maimonides based on the Torah and tradition.
What about the Ancient Egyptians or Zoroastrians? In both cases, these were monolatrous henotheistic faiths. This means that they had multiple divinities with powers not completely constrained by other divinities.
Ancient Persia:
There is no consensus on when Zoroaster lived. Zoroastrianism is unlikely to be older than Judaism, and in addition Zoroastrianism is by most standards a duotheism.
ancient Egypt:
In the case of Atenism, although all of the heavenly gods were reduced to Amun-Re, the Pharaoh was himself still held to be a god.
1) According to careful research, the original tradition, which was widespread, was monotheistic. However, it died out completely in a relatively brief span of time. This belief does not refer to a specific people, country, or named religion, since it was not centralized or organized.
2a) The first continuous monotheistic tradition and religion as we know it, was and is Judaism, the tradition founded by Abraham. It began 3800 years ago, and its founding principle is that God is One. This was well before the Egyptian king Akhenaten, who in any case (according to tradition) was influenced by Israelite beliefs.2b) Some might claim that monotheistic Judaism started later, as (for example) King Josiah made reforms in the late First Temple period. However, this is the view of secular academe, and flies in the face of tradition. Judaism had already been monotheistic for eight centuries by the time of Josiah. All he did was to eradicate the idolatrous influences of those Jews who had strayed from their own religion. This had happened repeatedly (such as with Jehoshaphat [2 Chronicles 17:6], and Samuel before him [1 Samuel 7:3-4]); and those who strayed into pagan practices never encompassed the entire people.
3) Zoroastrianism, which might be suggested as another early contender, is not such a clear matter. There is no consensus on when Zoroaster lived. Moreover, the Zoroastrians believed in two gods, not one. The Jewish Sages who collated the Talmud in the early centuries of the Common Era lived in Babylonia, witnessed the practices of the Zoroastrians, and recorded this fact (Talmud, Sanhedrin 39a). In addition, in Zoroastrianism:
Yes
The Jews; although what is known as Hinduism could also qualify (Hindus believe in a supreme God although they have many gods).
the hebrews
The animisim came first, then the polytheism and then the monotheism
They were the first group in the middle east to practice monotheism (or at least monolatry, which is an early form of monotheism).
yea they did
Henry More first used the word monotheism in a work published in 1660.
No, but her husband Akenarten did worship one god (the Arten) and this may be the first instance of monotheism.
Yes, you can. But, first you need to add them until they accept you. Most people will accept.
The Zoroastrians in ancient Persia are considered one of the first groups to adopt monotheism, with their belief in Ahura Mazda as the supreme deity.
At first people did not accept the work of the impressionists.
No, there were many major religions before it that taught monotheism. Judaism is an obvious example.