马吉安主义是由马吉安于144年所发展的神学思想，将二元论放在基督教的信仰中，是当时甚受欢迎的一种思想。他认为《希伯来圣经》内所记载的上帝与基督的父有所不同，且《希伯来圣经》内本身充斥着矛盾。他的思想是指《希伯来圣经》中的上帝是「创造的上帝」，是充满邪恶且暴力的上帝；基督的父则是温柔善良的上帝。他摒弃了《希伯来圣经》，且只集合了数卷使徒保罗及路加的著作成为正典。

由于马吉安主义没有任何著作留下来，包括相信是记载马吉安思想的《对比论》（Antitheses），他的主张及思想只有从反对他的人的著作中重新构筑出来。这些作者包括反对马吉安主义最要紧的特土良（着有五卷《反马吉安》）、爱任纽（着有《反异端》）等。在325年的第一次尼西亚公会议，马吉安主义被判为异端，而《圣经》的正典亦同时被确立。

Marcionism was an Early Christian dualist belief system that originated in the teachings of Marcion of Sinope at Rome around the year 144.

Marcion believed Jesus was the savior sent by God, and Paul the Apostle was his chief apostle, but he rejected the Hebrew Bible and the God of Israel. Marcionists believed that the wrathful Hebrew God was a separate and lower entity than the all-forgiving God of the New Testament. This belief was in some ways similar to Gnostic Christian theology; notably, both are dualistic, that is, they posit opposing gods, forces, or principles: one higher, spiritual, and "good", and the other lower, material, and "evil" (compare Manichaeism), in contrast to other Christian views that "evil" has no independent existence, but is a privation or lack of "good", a view shared by the Jewish theologian Moses Maimonides.