In his polemical treatise Dialogue with Trypho the Jew, published around 165, he describes their existence. In my view, his contemporaries marked the decisive phase in the birth of Christianity.

[S&W, p. OT 433] In the mid-2nd century, St. Justin Martyr in his Dialogue With Trypho commented on the difference between the Christian OT and the Hebrew Canon. Tertullian during this period also ...