The birthday of the church is Pentecost Sunday when the disciples first received the Holy Spirit.  Jesus had promised to send the Holy Spirit as a helper, comforter, advocate and guide.  Jesus had more to say, but didn't.  The Spirit of Truth was promised to guide us into all truth.  The Holy Spirit is still speaking to us the words of Jesus - both what Jesus said and taught, and what Jesus still has to say that isn't recorded in the gospels