catholics believe it is a sacrilege - a form of blasphemy - to receive communion while rejecting basic teachings/truths.
Yes, you are correct that it is wrong to take communion “unworthily” (without knowing Christ or without serious self examination and repentance). This is talked about in 1 Corinthians 11:27-34.
However, neither that Scripture nor any other Scripture says anything like “communion through the Catholic Church or any other specific church is NECESSARY for SALVATION.”
When the Bible talks about Salvation it has few requirements. Basically a person needs to recognize their sin nature, accept that Jesus is their only source for salvation and eternal life, repent of their sins (which means to turn away from them and not stay in the path of continuing them) and call on Jesus to save them. These things can be done without the Catholic Church and certainly without communion.
Romans 10:13 “For WHOSOEVER shall call upon the name of the Lord SHALL be saved.”
Communion is a serious rememberance of who Jesus is, what he has done, and who we are supposed to be like.