What is "to say amen to"

Amen \A`men"\ (?; 277), interj., adv., & n. [L. amen, Gr. 'amh`n, Heb. [=a]m[=e]n certainly, truly.] An expression used at the end of prayers, and meaning, So be it. At the end of a creed, it is a solemn asseveration of belief. When it introduces a declaration, it is equivalent to truly, verily. Note: It is used as a noun, to denote:

1. concurrence in belief, or in a statement; assent;

2. the final word or act;

3. Christ as being one who is true and faithful.

   And let all the people say, Amen.
   --Ps. cvi. 48.

   Amen, amen, I say to thee, except a man be born again, he can not see the kingdom of God.
   --John ii. 3. Rhemish Trans.

   To say amen to, to approve warmly; to concur in heartily or emphatically; to ratify; as, I say Amen to all.