Reject specifically Christian beliefs about Jesus
Christianity is likewise an Abrahamic monotheism, believing in the same single supreme God as its two Semitic cousins. However, the Christian faith also maintains certain uniquely characteristic religious claims about Jesus Christ, and this gives rise to another shared commonality between Judaism and Islam: they both flatly reject those uniquely Christian claims about Jesus. Such characteristically Christian beliefs as the divinity of Jesus, the resurrection of Jesus, and the unique role and status of Jesus as being a literal Son of God and a divine savior are all regarded by both Jews and Muslims alike as not only patently false, but even as downright blasphemous. Christianity maintains that Jesus is, in some sense, a divine incarnation of God himself — the second Person in a divine Trinity, consisting of Father, Son, and Holy Spirit — and hence coequal with, and even identical to, God himself. Judaism and Islam each see such radical claims as both nonsensical (in their view, God simply does not become incarnate as an individual human being), as well as idolatrous (by essentially equating a human being — in this case, Jesus — with the divine, making him coequal with or identical to God).
From the perspectives of both Judaism and Islam, such extreme beliefs about Jesus are seen as being utterly inconsistent with the sort of genuine, radical, and uncompromising monotheism upon which both Judaism and Islam absolutely insist, as non-negotiable core articles of faith. For Jews and Muslims alike, God is One, and not in any sense Three; for them, Christianity’s belief in the Trinity smacks of an unacceptable “tri-theism.” For their part, Muslims do regard Jesus as a very great prophet indeed, but as nothing more elevated or holy than that — a mere mortal, nothing more. Jews take an even dimmer view of the matter, seeing Jesus as at best a failed wannabe messiah, or at worst as a false prophet. In either case, for both Judaism and Islam alike, Jesus is no savior, is not divine, was never resurrected, and is not God incarnate (or the literal Son of God) in any sense whatsoever. Upon such matters, Jews and Muslims are in full agreement with each other.