Not that we are sufficient... — He had not used the word “sufficient” of himself, but it was clearly the implied answer to the question, “Who is sufficient for these things?” In the Greek there are two different prepositions for the one “of” in English. “Not as though we are sufficient of ourselves to form any estimate as originating with ourselves,” would be a fair paraphrase. The habit of mind which led St. Paul to emphasise the shades of meaning in Greek prepositions to an extent hardly to be expressed in English, and not commonly recognised, it may be, in colloquial Greek, is seen again in Romans 11:36.
Is of God. — The preposition is the same as in the second of the two previous clauses. The sufficiency flows from God as its source: originates with him.