A concise indication of the proof contained in this phrase:
Firstly: The answer to the question of who “those” are, is the verse:
Of the prophets [who teach], the sincere [lovers of the truth], the witnesses [or martyrs] [who testify], and the righteous [who do good].1
This verse lists the four classes of mankind who have received the bounty of right-guidance. That is, with “the prophets” it indicates Muhammad (Peace and blessings be upon him), who is the leader of the four classes, with “the sincere [lovers of the truth]” Abu Bakr the Veracious, and with “the witnesses [or martyrs] [who testify]” ‘Umar, Uthman, and ‘Ali (May God be pleased with them). With these prophecies, that after the Prophet Muhammad (PBUH) and after Abu Bakr, both ‘Umar, and ‘Uthman, and ‘Ali would be both Caliphs and martyrs, this verse displays a flash of miraculousness.
Secondly: The truth of Divine unity, which these most elevated, most true and rightly-guided four classes of mankind have taught with all their strength since the time of Adam, with innumerable proofs, miracles, wonders, evidences, and illuminations, and whom the majority of mankind have affirmed, is most certainly as definite as the sun. With their hundreds of thousands of miracles and proofs, these pre-eminent members of mankind have proved their veracity and truthfulness, and their consensus and agreement in positive matters such as Divine unity and the Creator’s necessary existence forms such a proof it can be in no way doubted. They are the straightest and the most veracious and verified of guides and the leaders in perfection among mankind, and man is the most important result of the universe, vicegerent of the earth, and of living creatures the most elevated and with the most comprehensive capacity. Those therefore who do not recognize a truth upon which those four classes have unanimously agreed, and deny the truths in which they believed unshakeably, with ‘absolute certainty’, ‘vision of certainty’, and ‘knowledge of certainty’, and in proof of which they put forward the universe together with all its beings, such deniers surely perpetrate an infinite crime and are deserving of infinite punishment.