The Word of God comes to us in various ways at various times. Sometimes it is in the still small voice, as it came to Elijah in 1 Kings 19, but that is not so uniform as to be considered the normative way that God speaks. God's Word also comes through Moses commanding Pharaoh, "Let my people go!" Or through John shouting from the wilderness, "Repent, for the kingdom of heaven is at hand!" Or through Paul rebuking the Galatians, "O foolish Galatians! Who has bewitched you? It was before your eyes that Jesus Christ was publicly portrayed as crucified." Don't be quick to limit how God must speak to his people without taking the whole of Scripture into account. The point of the quote from Clowney is that when a man is called to be a minister of the gospel, he must understand the weight of that call, because it comes with the need to preach with authority from God, "Thus saith the Lord." Sometimes those words will be soft words of comfort and consolation, but other times they will be commands to repent from wickedness. This is one reason why Reformed churches put a high premium on expository preaching, verse by verse, through the Bible. It puts things in proper balance, such that you don't have one minister who preaches nothing but hell-fire and brimstone all the time, while another just focuses on how much God loves us all the time. The whole of what Scripture says must be preached. Both words of comfort and assurances of love as well as warnings to repent and flee from sin and its wages.