Why may Muslims believe the Shahadah unites religious believers?

Muslims may believe the Shahadah unite religious believers due to Ibrahim uncovering monotheism in Christianity and Islam. Abraham was the first-ever human to reject the Islamic belief of polytheism for monotheism; though Ibrahim was neither a Christian nor a Muslim yet united the religions in their truth of only having one true God. The quote from the Quran, “Ibraham… was an upright man, the Hanif (non-religious though believing of Tawhid) who had surrendered (to Allah)” infers Muslims (and Christians) can only give themselves to God through being monotheistic. Therefore Muslims may believe the Shahadah unites religious believers through the oneness of God in Christianity and Islam.