{Beginning} (arxamenos). Aorist middle participle of
arch(935c), agreeing (nominative) with ho kurios I(8873)ous (the Lord
Jesus). The ministry of Jesus began with the ministry of John.
Strictly speaking arxamenos should be the accusative and agree
with martura (witness) in verse  22 , but the construction is a
bit free. The ministry of Jesus began with the baptism of John
and lasted until the Ascension. {A witness with us of his
resurrection} (martura t(8873) anastase(9373) autou sun h(886d)in). This
Peter considers the essential thing in a successor to Judas. The
one chosen should be a personal witness who can speak from his
own experience of the ministry, resurrection, and ascension of
the Lord Jesus. One can easily see that this qualification will
soon put an end to those who bear such personal testimony.