for a witness: i.e. with a view to bearing witness; not merely to be a witness. That would be martur (mart us, as in Act_1:8, Act_1:22, &c.) This is marturia = a bearing witness. Greek. eis.App-104. Not the same word as in Joh_1:16.
to bear witness = in order that (Greek. hina) he might bear witness. Greek martureo, a characteristic word of this Gospel. See note on p. 1611,
witness. Greek marturia, a characteristic word of this Gospel.
of = concerning. Greek. peri.App-104.
that = in order that. Greek hina. Often found in John.
all: i.e. all, without distinction.
through. Greek dia.App-104. Joh_1:1.
him. John the Baptist. Compare Joh_5:33. Act_10:37; Act_13:24.
believe. See App-150. A characteristic word of this Gospel. See note on p. 1611.