A.T. Robertson Commentary Hebrews 3:18

"And to whom sware he that they should not enter into his rest, but to them that were disobedient?" — Hebrews 3:18 (ASV)

That they should not enter (μη εισελευσεσθα). Negative μη (cf. ε in verse 11) and the future middle infinitive in indirect discourse.

To them that were disobedient (τοις απειθησασιν). Dative masculine plural of the articular first aorist active participle of απειθεω, active disobedience with which compare απιστιας in verse 12,19.