On the internal/external covenant membership distinction: De Doctrina Christiana, book 3, ch. 32
THE SECOND RULE OF TICHONIUS
The second rule is about the twofold division of the body of the Lord; but this indeed is not a suitable name, for that is really no part of the body of Christ which will not be with Him in eternity. We ought, therefore, to say that the rule is about the true and the mixed body of the Lord, or the true and the counterfeit, or some such name; because, not to speak of eternity, hypocrites cannot even now be said to be in Him, although they seem to be in His Church. And hence this rule might be designated thus: Concerning the mixed Church. Now this rule requires the reader to be on his guard when Scripture, although it has now come to address or speak of a different set of persons, seems to be addressing or speaking of the same persons as before, just as if both sets constituted one body in consequence of their being for the time united in a common participation of the sacraments.