This study of 1 John focuses particular attention on the description of sin as “lawlessness” (3.4) and on the warning against “idolatry” in the epistle’s concluding verse (5.21). There appears to be little consensus within Johannine scholarship in regard to the meaning of these opaque statements and their function within the letter’s overall message. This paper argues that the exegesis of these verses can be substantially clarified and their purpose within the epistle delineated much more precisely when they are read in light of the Book of Deuteronomy and its exposition of the subject of apostasy. It is argued that these Johannine statements are in fact to be understood as instances of Biblical intertextuality: the author of 1 John is intentionally “echoing” terms and concepts from the Book of Deuteronomy which support his stern warnings against the sin of apostasy (e.g. 5.16-17). The author of 1 John is thus re-appropriating Deuteronomy’s perspective on unbelief and apostasy for the early Christian communities, with specific application to their confession of Jesus as “the true God and eternal life” (5.20).