Heretics don’t follow the same practices and can be refuted therefore without the scriptures

These evidences, then, of a stricter discipline existing among us, are an additional proof of truth, from which no man can safely turn aside, who bears in mind that future judgment, when “we must all stand before the judgment-seat of Christ,” to render an account of our faith itself before all things.

On the present occasion, indeed, our treatise has rather taken up a general position against heresies, (showing that they must) all be refuted on definite, equitable, and necessary rules, without any comparison with the Scriptures.

Tertullian
Against Marcion
The Prescription Against Heretics
Chapter XLIV

Heretics lack discipline because they do not fear God

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)2296 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.

Tertullian
Against Marcion
The Prescription Against Heretics
Chapter XLIII