# what had been said through the prophet might be fulfilled, when he said

"what God told one of the prophets to write long ago might come true" 

# when he said

"when the prophet said"

# I will open my mouth

"I will speak" 

# things that were hidden

"things that God has kept hidden" 

# from the foundation of the world

"since the beginning of the world" or "since God created the world"