King James Bible Jeremiah 33:
1Moreover the word of the LORD came unto Jeremiah the second time, while he was yet shut up in the court of the prison, saying, 2Thus saith the LORD the maker thereof, the LORD that formed it, to establish it; the LORD is his name; 3Call unto me, and I will answer thee, and shew thee great and mighty things, which thou knowest not.
Who is being addressed in verse 3? To whom is the promise made?
All the pronouns are singular, referring to Jeremiah.
Does it apply to all future God's servants to come?
Four chapters earlier, God promised Jeremiah in 29:
10For thus saith the LORD, That after seventy years be accomplished at Babylon I will visit you, and perform my good word toward you, in causing you to return to this place. 11For I know the thoughts that I think toward you, saith the LORD, thoughts of peace, and not of evil, to give you an expected end. 12Then shall ye call upon me, and ye shall go and pray unto me, and I will hearken unto you. 13And ye shall seek me, and find me, when ye shall search for me with all your heart. 14And I will be found of you, saith the LORD: and I will turn away your captivity, and I will gather you from all the nations, and from all the places whither I have driven you, saith the LORD; and I will bring you again into the place whence I caused you to be carried away captive.
Here the pronouns are plural referring to the Israelites.
Both passages encourage people of God to pray to him. However, the resultant promises were different.
What does the promise actually mean?
To God's prophets, God promised to reveal them great and mighty things.
To God's people in general, God promised not to reject them if they would call on him.
This bears out even in the OT. Jesus says in John 6:
37b whoever comes to me I will never drive away.