To "declare" means to say something formally, publicly and with emphasis; you might say what you think, but you only declare things you're sure of. So just as saying relates to sharing truth at various levels, "declaring" relates to sharing fundamental, unquestioned principles, often those that reveal the Lord and the salvation that lies in His embrace.

In Amos 4:12-13, 'to declare' signifies to flow in.

(References: Apocalypse Revealed 478; Arcana Coelestia 7550, 8762)