Herein (en toutōi). His whole confession of belief in Act 24:14, Act 24:15.
Do I also exercise myself (kai autos askō). “Do I also myself take exercise,” take pains, labour, strive. Old word in Homer to work as raw materials, to adorn by art, then to drill. Our word ascetic comes from this root, one who seeks to gain piety by rules and severe hardship. Paul claims to be equal to his accusers in efforts to please God.
Void of offence (aproskopon). This word belongs to the papyri and N.T. (only in Paul), not in the ancient writers. The papyri examples (Moulton Milligan, Vocabulary) use the word to mean “free from hurt or harm.” It is a privative and proskoptō (to cut or stumble against). Page likes “void of offence” since that can be either active “not stumbling” as in Phi 1:10 or passive “not stumbled against” as in 1Co 10:32 (the first toward God and the second toward men), the only other N.T. examples. Hence the word here appears in both senses (the first towards God, the second towards men). Paul adds “alway” (dia pantos), a bold claim for a consistent aim in life. “Certainly his conscience acquitted him of having caused any offence to his countrymen” (Rackham). Furneaux thinks that it must have been wormwood and gall to Ananias to hear Paul repeat here the same words because of which he had ordered Paul to be smitten on the mouth (Act 23:1.).