"doctrinae index disciplina est."

"In their discipline we have an index of their doctrine."

     Tertullian, De praescriptione haereticorum 43, as trans. Holmes (ANF 3).  See, for the Latin, p. 33 of the edition ed. Preuschen (2nd ed., Tübingen, 1910), which I'm not claiming is the best, necessarily; just handy.

An/The "index of (the) doctrine is (the) discipline." 

Holmes, again: 

from the very nature of their conduct, may be estimated the quality of their faith. In their discipline we have an index of their doctrine. They say that God is not to be feared; therefore all things are in their view free and unchecked. Where, however is God not feared, except where He is not? Where God is not, there truth also is not. Where there is no truth, then, naturally enough, there is also such a discipline as theirs. But where God is, there exists 'the fear of God, which is the beginning of wisdom.' Where the fear of God is, there is seriousness, an honourable and yet thoughtful diligence, as well as an anxious carefulness and a well-considered admission [to the sacred ministry] and a safely-guarded communion, and promotion after good service, and a scrupulous submission (to authority), and a devout attendance, and a modest gait, and a united church, and God in all things.

     I was put onto this by Geoffrey Wainwright ("Heresy then and now: reflection on a treatise of Tertullian," Pro ecclesia 13:2 (Spring 2004):  220), and for that reason haven't mastered the larger context.  See also DS, sv Disciplina, vol. 3 (1957), col. 1292 (where disciplina is said to connote "the type of life that the Christian faith requires, the morality that is conformed to this [doctrina] and is the criterion of it"), and the scholarship cited there and of course published since.