“Now faith is unwavering assent to what is heard [from Christ], in full assurance of the truth of what is proclaimed by the grace of God. This was shown by what what was testified to Abraham, that ‘he did not waver in unbelief, rather he was strengthened in faith, giving glory to God, and was fully assured that what he promised he is able to do.’ But if ‘the Lord is faithful in all his words’ and ‘all his commandments are faithful, established unto ages of ages, made in truth and uprightness,’ it is a clear indictment of abandoning the faith and of arrogance either to supplant anything that is written or to introduce anything not written. For our Lord Jesus Christ said, ‘My sheep hear my voice,’ and before this he said likewise, ‘A stranger they will not follow but will flee from him, because they do not know the voice of strangers.’ And the Apostle, using an example from human affairs, more emphatically forbids adding or subtracting anything in the God-breathed Scripture, which he has in mind when he says, ‘Though a covenant be confirmed by human agency, no one denies it or makes addition to it.”
-St. Basil the Great, ‘On the Faith’, page 73 in his On Christian Ethics. Similarly, §26.1 of Basil’s ethics, “That it is necessary to confirm every word or matter with the testimony of the God-breathed Scripture, so that the good is established and the evil reproached.” He cites Matthew 4:3-4 and Acts 2:12-17ff for this latter rule.
This is not only sola scriptura, but the regulative principle. Faith in Christ includes faith in his words, which cannot be subtracted from or added to in faith or practice without modifying faith in Christ. Ethical conduct in faith demands that all faith and practice (word or matter) be ruled by God speaking in scripture: the Bible is the norming norm, because “It is necessary not to be fixed on one’s own reasonings to the rejection of what is said by the Lord, but to understand that the words of the Lord are worthier of belief than one’s own convictions” (§8.3). This is the practice of faith.