Gnosticism is basically Christianity with extra secret knowledge: a legitimate spiritual path that teaches you can find God within by learning hidden truths

Gnostic systems (the term covers a range of related but distinct movements, including Valentinianism and Sethianism, rather than one uniform doctrine) share a small set of core commitments that put them at odds with orthodox Christianity at the most foundational level, not at the margins. First: the god who created the material universe, the Demiurge, often identified by Gnostic writers with the God of the Old Testament, is not the true, highest God. Gnostic texts, including material preserved in the Nag Hammadi library (such as the Apocryphon of John), typically portray the Demiurge as ignorant, arrogant, or actively malevolent, a lesser and flawed being who mistakenly believes himself to be the ultimate deity. The true, transcendent God exists entirely beyond and apart from the material cosmos the Demiurge made.

Second, this reshapes what "salvation" even means. Because the material world itself (including the human body) is regarded in most Gnostic systems as a flawed or corrupt production of the Demiurge, salvation is not about redemption of the body or of creation, as in Christian teaching on bodily resurrection (see, for example, 1 Corinthians 15). It's about escape: freeing a trapped spark of divine spirit that has fallen into matter, through gnosis (direct, esoteric knowledge of one's true divine origin) rather than through faith, repentance, or the atoning death of Christ understood as dealing with sin within a good creation. Salvation in Gnostic texts is fundamentally a matter of correct knowledge, available to an initiated few, rather than a matter of grace offered publicly to all.

Orthodox Christianity, defined by the early creeds and the apostolic writings later collected in the New Testament, holds a position that is a near-total inversion of this: one God created the material universe and called it good (Genesis 1:31), that same God is the God revealed in Jesus Christ (there is no split between a flawed Creator-god and a separate true God), salvation involves the resurrection of the body rather than escape from it, and the good news is proclaimed publicly to all rather than reserved as secret knowledge for spiritual elites. Early Christian writers who opposed Gnostic teaching, most notably Irenaeus of Lyons in his Against Heresies (written around AD 180), framed the dispute in exactly these terms: arguing that Gnostic teachers had substituted a fundamentally different God and a fundamentally different account of matter and salvation, not merely added supplementary teaching to the apostolic faith.