The Opening of the Mouth ceremony is found in The Book of the Dead, which is a book of spells and instructions for the deceased on how to appease the gods and function in the afterlife. This particular ceremony was performed on the mummy to cleanse the deceased and help with the successful transition into the afterlife. This successful transition allows the deceased to open their mouth to use the correct and needed words to navigate through the afterlife, and be able to be unbound and use food and clothing.
According to Egyptian mythology, the heart was where all thought, personality, and other governing functions of a person originated. The heart also held a record of all good and evil that a person had done, and it was weighed against an ostrich feather to gauge purity and worthiness to enter the afterlife. Finding the heart to weigh more than the feather meant that the person had sinned in their life and the heart was then consumed by the amalgamated beast Ammit. Having the heart consumed was undesirable as this meant that the life cycle as exemplified by Osiris would end for the deceased.
In the scene depicted, Ani and Tutu are real people waiting to be judged by the mythological god Anubis for acceptance into the afterlife. Ammit can be seen waiting behind Anubis in position to consume the heart if found unworthy. Across the top of the image is a representation of the 12 gods that are sitting in as witnesses and Thoth can be seen preparing to record the verdict from Anubis.
Osiris became important to the funerary rites due to the conflict between himself and Seth. Through the multiple times that Seth killed Osiris and Isis brought him back to life, Osiris gained in significance until he absorbed the essence of Khentiamentiu, Foremost of Westerners, and became Osiris, Foremost of Westerners. The deceased in the funerary rites is called Osiris in honour of the life cycle that Osiris displayed, by being born, living, dying and then being resurrected by Isis. In calling the deceased by Osiris, the Egyptians were hoping to ensure the deceased’s ability to continue on this life cycle.
Chapter 125 of the Book of the Dead indicates that the deceased when in the Hall of Two Truths is mainly focused on the 48 gods that reside there finding him to be honest and truthful in his statements. The deceased is also demonstrating that they never disrespected or offended any of the gods while alive, as well as was kind to his fellow man and to the earth. There are a number of items listed in the Book of the Dead which were deemed immoral to have committed, such as robbing the poor. The Egyptians also believed that the deceased had an effect on the successes of the rest of their family depending on how moral they presented themselves in the Hall of Two Truths
D'Auria, Sue; Lacovara, Peter; Roehrig, Catharine H. 1988. "Osiris" in Mummies and Magic: The Funerary Arts of Ancient Egypt. pg. 50-51
D'Auria, Sue; Lacovara, Peter; Roehrig, Catharine H. 1988. "Funerary Texts and Their Meaning" in Mummies and Magic: The Funerary Arts of Ancient Egypt. pg. 38-49
Lichtheim, M., Ancient Egyptian Literature, vol 1. 1975-1980 Berkeley
The Book of the Dead
 D’Auria, Funerary Texts and Their Meaning, p. 41,43
 Lichtheim, Literature vol 1
 The symbol of Maat who was over truth and purity.
 Westerners here mean the dead; thus ‘Foremost of the Westerners’ means the leader or god of the dead.
 D’Auria, Osiris, p. 51
 Book of the Dead