At the baptism of Jesus the heavens were opened (Matt 3:16-17). As a part of that experience the foreordained sign of the dove was given, and the voice of the Father out of the heavens was heard bearing record of His beloved Son.
Nephi corroborates the account given in Matthew of Christ's baptism (1 Ne. 11:27). He learned these things when he himself beheld "the heavens open" (1 Ne. 11:14). In fact, the Book of Mormon begins with Lehi's experience in beholding "the heavens open," wherein he saw God sitting upon His throne surrounded by numberless concourses of angels (1 Ne. 1:8). This is a key, given in the first chapter of the book.
Each account in scripture in which we read about the heavens opening gives to us only a small part of the experience that actually unfolded. The words in scripture do not fully capture the experience. The uninitiated are invited to come and partake, without having the mysteries unfolded to them in the words of a book. We are to seek further light and knowledge.
Were the sign of the dove and the Father's voice only seen and heard by Christ? Was John also a privileged witness of these things? Did he behold the heavens open? If so, what does that mean? What does it mean to behold the heavens open?