In this article it is argued that Justin’s attitude toward the Mosaic Law is based on three fundamental factors all of which were based on earlier Christian tradition: Firstly, it was based on the prophetic prediction that the old Sinaitic covenant would be substituted by the new one, i.e. on Jer 31:31–34 which was an important key-text already used in the New Testament. Secondly, many Old Testament references to the hardness of the people’s heart against listening to the word of God was developed in the Second Temple Jewish texts to address the Jewish people’s unwillingness to listen to the word of God. In Christian theology this topic was developed as a reaction to the Jews’ unwillingness to accept the Christian message. Thirdly, Justin followed an apostolic tradition that Jewish believers in Jesus (“Nazoraeans”) have the right to continue to practise the Mosaic Law. Justin received all three of these topics from older Christian traditions and made a synthesis of them. This explains why there is a certain tension in Justin’s theology.