Because it is contained in Scripture, "Behold, I lay in Zion a chief cornerstone, chosen, and precious: He who believes in him will not be disappointed."
But the words of the Prophet may be taken in two ways, either as a promise or as an exhortation. The future time is referred to, “He shall not make haste;” but in Hebrew the future is often to be taken for an imperative, “Let him not make haste.” Thus the meaning would be, “Be ye not moved in your minds, but quietly entertain your desires, and check your feelings, until the Lord will be pleased to fulfill his promise.” So he says in another place, 