For they that are after the flesh do mind the things of the flesh; but they that are after the Spirit the things of the Spirit.
Note 11 at Ro 8:5: Ro 8:5-8 explains why only those who walk after the Spirit (see note 10 at Ro 8:4) are seeing the righteousness of the Law fulfilled in their lives (see note 9 at Ro 8:4). It is because whatever people think on is what they are going to become or do (Pr 23:7). Those who are after the flesh think on carnal things and therefore do carnal things. Thinking carnally can only produce death, while thinking spiritually (according to the Word, Joh 6:63) can only produce life (see note 15 at Ro 8:6).
Note 12 at Ro 8:5: This verse gives us a test so that we can determine if we are walking after the flesh or after the Spirit: We just have to judge what we are thinking about. If we are consistently thinking on the things of the Spirit (Joh 6:63), then we are walking after the Spirit. If we are dominated with carnal thoughts (see note 13 at Ro 8:6), then we are walking after (see note 20 at Ro 8:9) the flesh (see note 3 at Ro 7:18).