The Apostle is the religious leader in Tellius and, together with the senate, decides how Begnion is ruled. During the time of Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn, Sanaki is the apostle of Begnion, but in time it is discovered that she is not the true apostle.
The apostle can hear the voice of The Goddess, which is why she is Begnion's spiritual leader. Sanaki had been lead to believe she was too young to hear the goddess's voice, so she never doubted her identity as the true apostle until Vice-Minister Lekain told her the truth: That she was not the first-born granddaughter of the former apostle Misaha and that she had an older sister who was the true apostle of Begnion.
It is revealed in a second playthrough of Radiant Dawn that the powers of the apostle do not come from Altina, but rather her husband, Lehran. Altina's granddaughter, Yoram was the first to inherit Lehran's power to speak with the goddesses and the first recognized apostle of Begnion.
Since then, every true apostle has been branded; members of the imperial line that did not inherit the brand, like Sanaki, have no power beyond an ordinary beorc.