Samuel spoke by the power of the Holy Ghost and thus with what Nephi called “the tongue of angels.” He spoke “the words of Christ” (2 Nephi 32:2-3).
That is, he spoke what Christ wanted spoken, what the Master himself would have said had he been personally present. Samuel’s was a divine investiture of authority; he, as the agent of God, spoke in behalf of his Principal.
“What I the Lord have spoken, I have spoken,” Christ declared in this dispensation, “and I excuse not myself; and though the heavens and the earth pass away, my word shall not pass away, but shall all be fulfilled, whether by mine own voice or by the voice of my servants, it is the same”
(D&C 1:38).
At the time of the organization of the restored Church, Christ spoke similarly: “Wherefore, meaning the church, thou shalt give heed unto all his [Joseph Smith’s] words and commandments which he shall give unto you as he receiveth them, walking in all holiness before me; for his word ye shall receive, as if from mine own mouth, in all patience and faith” (D&C 21:4-5).