Who were Messengers?

Allah sent his Messengers in all nations:

35:24 Surely We have sent you with the truth as a bearer of good news and a warner; and there is not a people but a warner has gone among them.

[2:213] Mankind were one community, then they differed among themselves, so ALLAH raised Prophets as bearers of good tidings and as warners, and sent down with them the book containing the truth that HE might judge between the people wherein they differed. But then they began to differ about the Book, and none differed about it except those to whom it was given, after clear Signs had come to them, out of envy towards one another. Now has ALLAH, by HIS command, guided the believers to the truth in regard to which they (the unbelievers) differed; and ALLAH guides whomsoever HE pleases to the right path.

All Messengers were Muslims:(Monotheist)

22:78And strive in His cause as ye ought to strive, (with sincerity and under discipline). He has chosen you, and has imposed no difficulties on you in religion; it is the cult of your father Abraham. It is He Who has named you MUSLIM, both before and in this (Revelation); that the Apostle may be a witness for you, and ye be witnesses for mankind! So establish regular Prayer, give regular Charity, and hold fast to God! He is your Protector - the Best to protect and the Best to help.

42:13 The SAME religion has He established for you as that which He enjoined on Noah - the which We have sent by inspiration to thee - and that which We enjoined on Abraham, Moses, and Jesus: Namely, that ye should remain steadfast in religion, and make no divisions therein: to those who worship other things than God, hard is the (way) to which thou callest them. God chooses to Himself those whom He pleases, and guides to Himself those who turn (to Him).

35:32 We passed the scripture from generation to generation, and we allowed whomever we chose from among our servants to receive it. Subsequently, some of them wronged their souls, others upheld it only part of the time, while others were eager to work righteousness in accordance with GOD's will; this is the greatest triumph.

Example of what prophet Joseph (a.s.) said this to people in prison

12:38 "And I followed instead the religion of my ancestors, Abraham, Isaac, and Jacob. We never set up any idols beside GOD. Such is the blessing from GOD upon us and upon the people, but most people are unappreciative.

12:39 "O my prison mates, are several gods better, or GOD alone, the One, the Supreme?

12:40 "You do not worship beside Him except innovations that you have made up, you and your parents. GOD has never authorized such idols. All ruling belongs to GOD, and He has ruled that you shall not worship except Him. This is the perfect religion, but most people do not know.