"Sin crouches at the door"
A verse in the Torah states that negativity and evil forces hover by doorways, openings, and beginnings of all kinds. This idea is related to the secret and power of a seed. If one plants a defective apple seed, it will yield a defective apple tree. Doorways and beginnings represent the seed level. The door to the home is the seed of the entire house. Negative forces attack at the seed level so as to influence all the future stages and developments. They cling to all entranceways to infect the seed with negativity. The Mezuzah, or door post ornament, not only cancels this negative force, but also transforms negative energy into positive energy. The Mezuzah contains a piece of parchment bearing the Hebrew letters Shin, Dalet, and Yud שדי. This is a powerful Name of God that brings us protection.
These passages bring protection to all the starts or beginnings in our lives, including marriage, business ventures, or any other area of activity.