'Declare' in the Bible

And I declare to you that you are Peter, and that upon this Rock I will build my Church, and the might of Hades shall not triumph over it.

"Because your faith is so small," He replied; "for I solemnly declare to you that if you have faith like a mustard-seed, you shall say to this mountain, 'Remove from this place to that,' and it will remove; and nothing shall be impossible to you.

In that case how are the Scriptures to be fulfilled which declare that thus it must be?"