As I sware (hōs ōmosa). “Correlating the oath and the disobedience” (Vincent). First aorist active indicative of omnuō, old verb for solemn oath (Heb 6:13).
They shall not enter (ei eiseleusontai). Future middle of eiserchomai with ei as an anacoluthon for the Hebrew im (not). Really it is a condition of the first class with the conclusion not expressed, common in the lxx as here (Robertson, Grammar, p. 1024).
Into my rest (eis tēn katapausin mou). Old word from katapauō (Heb 4:8), to give rest, in lxx, in N.T. only in Act 7:49; Heb 3:11-4:11. Primarily the rest in Canaan and then the heavenly rest in which God dwells.