Yes, improvisation is essential. In a service, improvisation can always replace literature, but not the other way around.
It is key that you can improvise choral harmonizations, not only for accompanying congregational singing, but also for the other musical moments. With chorals, you can bring the theme of the service to the minds of the churchgoers, and thus prepare for and answer to the message of the sermon.
Chorals provide a solid and reliable inspiration for many kinds of improvisation and allow endless variation.
So my question to you is, do you know how to improvise simple choral harmonizations (4 voices, avoiding quint and octave parallels)? Perhaps we can give a few more tips depending on your answer.