Religious Vows

The most powerful gods are known to have some sense of when they are being mentioned, especially during formal and ritualized prayers. Though no guarantees are made to whether they answer, this gives some power to making a religious vow. It is no idle promise - it is an oath that may be overseen and enforced by a god. Though each god tends to enforce their follower's vows to different extents and in different ways, it is seen by most to be a fairly serious act.
For most, making a vow typically includes three parts:

A promise to do something, such as loyalty, faith, or some other act like defeating an enemy of the god. This might have stipulations attached.

A request for aid, such as protection, guidance, or some other act.

A penalty to be enforced if the promise is not upheld. This can be quite specific, or incredibly vague, depending on the god.

A particularly common but rather harsh penalty is the matter of a mortal's soul. For gods that promise them a place in their domain, not upholding a vow could mean forfeiting that right. For others, it might mean that their soul isn't protected after they die, leaving it vulnerable to more nefarious creatures.

Though very uncommon, taking vows in the name of another god can sometimes be feasible. This usually depends on what is being promised, and the potential penalty. Promising your soul to multiple powerful entities tends to be a poor plan, as is making false vows.