What was the attitude of the Medieval Period/Early Renaissance Period Catholic Church toward the use of musical instruments in worship services? Why did they feel this way?

Sacred music in the Middle Ages was exclusively vocal. They refused musical instruments. Sacred music was solemn, strict, completely different from secular music. There was no division into musical measures, and no fixed rhythm.

Intuitism limited the use of instruments exclusively to support the singing of words, so that their meaning was well understood by listeners, and the souls of believers were sent to contemplate spiritual objects.