I will still argue that the church began in Acts 2 simply because God decides when the church begins not when Paul states "lo, we turn to the Gentiles." The argument is a simple one:
My argument is based upon the (unique) baptizing work of the Holy Ghost. The LORD Jesus Christ had spoken of this work of the Holy Ghost just before His ascension in Acts 1:5 as being yet future and unlike anything they had previously experienced saying For John truly baptized with water;
but ye shall be baptized with the Holy Ghost not many days hence. Although Acts 2 does not expressly record that the baptism of the Holy Ghost occurred on the Day of Pentecost, it is said in Acts 11:15, 16 that it did happen on that day in fulfillment of the promise of the LORD as recorded in Acts 1:5. It is later within the Pauline epistles that we find fully revealed the doctrinal significance of this baptism that unites believers with Christ in to His Body (1 Corinthians 12:13 For by one Spirit are we all baptized into one body, whether we be Jews or Gentiles, whether we be bond or free; and have been all made to drink into one Spirit.). So in other words, on the Day of Pentecost men were first placed into the Body of Christ and since the church is the Body of Christ (Colossians 1:18 And he is the head of the body, the church: who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.), the church could not have begun until Pentecost and therein from the evidence presented we can safely conclude that the church began on the Day of Pentecost.
Taken from:
http://www.christianforums.com/showthread.php?p=9386927#post9386927