Isaiah 50:4

Here ‘tongue’ is a picture word, to refer to the Servant’s authority to speak God’s message. The Servant qualifies to be God’s *messenger, because God has taught him what to say (see Jeremiah 1:9; Ezekiel 3:27).

Moreover, God had comforted his special Servant. So the Servant can also speak from personal experience (see Isaiah 40:1-2).