Charles Ellicott Commentary Romans 8:12

"So then, brethren, we are debtors, not to the flesh, to live after the flesh:" — Romans 8:12 (ASV)

We are debtors.—We are under an obligation. Observe that in the lively sequence of thought the second clause of the antithesis is suppressed, “We are under an obligation, not to the flesh (but to the Spirit).”

On verses 12-17:

These verses form a hortatory application of the foregoing, with further development of the idea to live after and in the Spirit.