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."
and he that believeth on him shall not be confounded: It was important to the apostle Peter, and it is important even today. Though we are not facing persecution as they faced, we have our own particular brand of trials to face. The saints can be calm in soul, calm and unflurried in life. If they are going to manifest Christianity, they must be not confounded: calm and unflurried in soul. They will never be put to shame in trusting Christ.