It is a common linguistic trend for the etymological roots of farewell/goodbye in many languages to include direct reference to, if not the name of god, then the idea of god. So leave room for the holy ghost because apparently god is especially happy to inhabit the space between people rather than intrude upon any warm social circles.
Consider adios, and it should be apparent to any Spanish speaker that the word is simply a combination of a and dios. It actually comes from the farewell, a dios vos acomiendo: I commend you to God. While it certainly seems nice to wish a friend the protection of god as they leave, adios still retains a certain morbidity; maybe there will not be another meeting in this lifetime. Also, the directly related French farewell, adieu (a dieu vous commant) can be traced to 1374, a time that was fertile ground for the oral tradition of such dark fairy tales we later see recorded by The Brothers Grimm. Considering the difficulties inherent to survival during the dark ages, it is no surprise that every farewell need be a blessing to carry a soul beyond this life.
Of a slightly different nature is the Japanese, sayonara, being a combination of sayo "that way," and nara "if," which when translated becomes something like, "if it is to be that way." While this can be connected to the divine through the total acceptance of suffering in life that is part of the Buddhist philosophy, it can be argued that this philosophy was a part of Japanese life long before the introduction of Buddhism from the mainland.
The conclusions to be drawn from this idea may only be of interest to fervent historians with a linguistic bent, but the mixing of god with history does tend to be popular subject matter today. With that in mind, I bid you adieu and goodbye*.
*1591, from godbwye (1573), itself a contraction of God be with ye