3:5 sufficient of ourselves. Compare II Corinthians 2:16.

3:6 new testament. Paul is no doubt referring to his very specific call by God, as recorded in Acts 9:3ff, and further in Acts 22:14-16 and Acts 26:16-18. The “new testament, of course, refers to God’s “new covenant,” as contrasted with His “old covenant,” as made with Moses and the children of Israel. Compare Hebrews 9:15 and following.

3:6 spirit. This contrast between the “letter” and the “spirit” does not refer to a supposed superiority of “spiritual interpretation” over “literal interpretation” of Scripture. While it may be profitable to draw occasional spiritual or allegorical or analogical applications from Scripture, these must always depend for any validity they may have on the basic literal accuracy and truthfulness of the written text. Otherwise, the meaning simply reflects the bias of the interpreter, rather than the intent of the writer. Whenever the writer intended to use a figure of speech to convey his meaning, he always made this clear in the context. Remember also that the human writers were writing under inspiration of the Holy Spirit, whose intent was to reveal God’s Word, not hide it.

3:6 letter. In context here, the “letter” is referring to the written Law, inscribed on tables of stone. This Law is “holy, and just, and good” (Romans 7:12), but in and of itself can only condemn sinners, not save them, for “by the law is the knowledge of sin” (Romans 3:20), and “all have sinned” (Romans 3:23).