Ἀφιλάργυρος ὁ τρόπος, ἀρκούμενοι τοῖς παροῦσιν. αὐτὸς γὰρ εἴρηκεν· οὐ μή σε ἀνῶ οὐδ᾽ οὐ μή σε ἐγκαταλίπω,The context is an encouragement to godly contentment. The reason that our lifestyle should not be that of silver-lovers is given as the impossibility of the Lord's abandoning us.
First you note the emphatic introduction. The simple verb εἴρηκεν is strengthened by the αὐτὸς. It is He, no less than He and none other than He, who has said this. Also, the verb is in the perfect tense: εἴρηκεν. He has gone on record, He has made a statement, the statement stands.
But then notice the piling up of negatives. First there are two (οὐ μή), and then there are three (οὐδ᾽ οὐ μή). Not, not, nor not not.
But I learn from Wallace that even the tense strengthens the negation, as these emphatic denials are paired with subjunctives. Hear him from p. 468:
One might think that the negative with the subjunctive could not be as strong as the negative with the indicative. However, while ouv + the indicative denies a certainty, ouv mh, + the subjunctive denies a potentiality. The negative is not weaker; rather, the affirmation that is being negatived is less firm with the subjunctive. ouv mh, rules out even the idea as being a possibility....So perhaps a "dynamic" rendering, a paraphrase, would be: : "for He Himself has said, 'There is no conceivable way I will leave you, nor is there any conceivable way I will abandon you.'"
At any rate, it is a wonderful promise, and is well captured in the wonderful hymn How Firm a Foundation:
The soul that on Jesus has leaned for repose,All praise and glory to our faithful, self-committed, oath-keeping God.
I will not, I will not desert to its foes;
That soul, though all hell should endeavor to shake,
I’ll never, no never, no never forsake.