God is eternal; God's Truth is eternal.  You say that not a single teaching of the church is the same as it was at the time of Pentecost.  That's simply not true. 

We still believe in the Incarnation, don't we?  
We still believe in Christ's resurrection, don't we?  
We still believe in Pentecost, don't we?  
We still believe Jesus is God, don't we? 
We still believe that God so loved the world that he gave his only begotten son, so that    everyone who believes in him will have eternal life, don't we?  
We still believe that we were created in God's image to know, love, and serve him, don't we? We still believe we should love one another, don't we?

This constant change of belief leads to the absurd conclusion that the only TRUE things are the things that happened most recently.  Who knows what will be real tomorrow......?