But I call God for a witness upon my soul (Egō de martura ton theon epikaloumai epi tēn emēn psuchēn). Solemn attestation, “calling heaven to witness is frequent in literature from Homer onwards” (Plummer). Thus God is described above (cf. 1Th 2:5, 1Th 2:10; Rom 1:9; Gal 1:20; Phi 1:8).
To spare you (pheidomenos humōn). Present middle participle (causal rather than final) of pheidomai, old verb, to hold back, to spare. Ablative case humōn.