the importance of one word

"let" implies a choice.   It is an intentional act on our part to allow, let, permit, open to, the peace of Christ be the primary rule, authority, measurement, of our lives.  God does not force anything upon us, no matter what the bad theology of suffering might say, or what the individualistic self-sufficency of our culture may suggest.  We get to choose whether or not to permit a higher authority into our lives.