{Ye know} (humeis oidate). Peter reminds his Gentile
audience that the main facts concerning Jesus and the gospel were
known to them. Note emphatic expression of humeis (you).
{Beginning} (arxamenos). The Textus Receptus has arxamenon
(accusative), but the nominative is given by Aleph A B C D E H
and is certainly correct. But it makes a decided anacoluthon. The
accusative would agree with 
h(886d)a used in the sense of message
or story as told by the disciples. The nominative does not agree
with anything in the sentence. The same phrase occurs in  Lu
23:5 . Here is this aorist middle participle almost used like an
adverb. See a similar loose use of arxamenos in the same sense
by Peter in  Ac 1:22 . The baptism of John is given as the
_terminus a quo_. The story began with a skip to Galilee after
the baptism just like the Gospel of Mark. This first message of
Peter to the Gentiles ( 10:37-44 ) corresponds in broad outline
with Mark's Gospel. Mark heard Peter preach many times and
evidently planned his Gospel (the Roman Gospel) on this same
model. There is in it nothing about the birth and childhood of
Jesus nor about the intervening ministry supplied by John's
Gospel for the period (a year) between the baptism and the
Galilean Ministry. Peter here presents an objective statement of
the life, death, and resurrection of Jesus with proof from the
Scriptures that he is the Messiah. It is a skilful presentation.