The Belief of Tawheed (Monotheism) is the original religion of humanity and the Fitrah (i.e., sound disposition and innate inclination) with which Allah has imbued all human beings. He Says (interpretation of meaning): So direct your face toward the religion, inclining to truth. Adhere to the Fitrah of Allah upon which He has created all people. No change should there be in the creation of Allah. That is the correct religion, but most of the people do not know. Quran 30:30 In this context, Ibn Katheer said: (Allah, Exalted be He, embedded in His creation the Fitrah to know Him and believe in His unique Oneness and that none is worthy of worship except Him).  See: ((Tafseer Ibn Katheer)) (6/313).
Allah, Exalted be He, created Adam, the father of humankind, and infused in him the sound creed (of Tawheed); he was a monotheist who believed in the due veneration and obedience to Allah, The Almighty.
Ibn Taymiyyah said: (Shirk was never the default faith among mankind, rather Adam and those who followed his religion, from among his descendants, were monotheists who complied with the guidance of prophethood. Allah, Most High, says (interpretation of meaning): And mankind was not but one community united in religion, but then they differed. Quran 10:19  See: ((Majmoo' Al-Fataawa)) (20/106) and ((Mabaahith Fee 'Aqeedat Ahlu-Sunnah Wal-Jamaa'ah)) by Naasir Al-'Aql (p. 19).
Allah has taken from all the descendants of Adam a pledge and made with them a covenant that He is their Lord and made them witness to it upon themselves. Allah, Most High, says (interpretation of meaning): And mention when your Lord took from the children of Adam - from their loins - their descendants and made them testify against themselves, saying to them, "Am I not your Lord?" They said, "Yes, we have testified." This - lest you should say on the day of Resurrection, "Indeed, we were of this unaware.” Or lest you say, "It was only that our fathers associated others in worship with Allah before, and we were but descendants after them. Then would You destroy us for what the falsifiers have done?" Quran 7:172-173
All people are born imbued with the sound Fitrah and develop upon it, as long as evil urges and temptations do not distract them away from it.
In his explanation of His saying, “all My slaves as Hunafaa’”, Al-Nawawee said: “It means ‘Muslims (adherents to Tawheed)’ … As for His saying, ‘then the devil lured them out of their true religion,’ it means that they fooled them into disbelief, led them astray, and ushered them into falsehood.”  See Sharh Al-Nawawee 'ala Muslim, 17\197
Aboo Al-‘Abbaas Al-Qurtubee said: “The Hadeeth means that Allah created the hearts of the children of Adam preconditioned to accept the truth, just like He created their eyes and ears with the infused ability to see and hear, so as long as it remains upon that condition and with that innate inclination it will recognize (and accept) the truth, and Islam is the true religion of Allah.”  See Al-Mufhm Lima-Ushkila min Kitaab Talkhees Muslim 6\676