1 John 1:3
That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ.
The apostles did not keep what they knew or had seen about Christ to themselves. They realized that the basis for all fellowship was found there and so they declared it freely and fully. All who receive the testimony of the apostles have fellowship with God the Father, His Son Jesus Christ, and with all other believers.