«The knowledge of the truth» is an important technical term in the Pastoral Epistles. It represents the basic elements of authentic (orthodox) Christianity - «being Christian» - and plays a major role in their polemic against false teaching. «The knowledge of the truth» also appears in the Coptic Book of Thomas from Nag Hammadi as a technical term representing its basic view of authentically «being Christian». The «Christian doctrine» of The Book of Thomas has striking similarities to the false teachings the Pastorals attack. It is suggested that The Book of Thomas may be a witness to the continuation, now seen from the heterodox side, of the struggle reflected in the Pastorals' polemic use of «the knowledge of the truth» to represent authentic (orthodox) Christianity.