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."
Whereunto also they were appointed ...  This does not mean that God foreordained, or appointed certain individuals to fall; but it means that God has finally and irrevocably appointed all disobedient souls to stumble. When the proud hierarchy of the ancient Israel refused to believe in Christ, they thereby thrust themselves under the blanket indictment of all unbelievers; and they fell, as God had ordained and appointed all unbelievers to fall.  The indictment still stands, and unbelievers still incur the wrath of God through their unbelief.