You are speaking of many different things that are broadly involved in the interpretation of the phrase that I have chosen for discussion, but what exactly do you mean when you speak of "partaking of" demons?
I think it is not possible to restrict the discussion strictly to verse 20. To find out what is meant by "partaking of" demons, we have to look back a few verses.
In v.14, Paul tells us, in view of Israel's experiences in the wilderness, to flee from idolatry. In v.16, he speaks of the Lord's supper and declares that is the
koinonia of the blood and body of Christ. Whatever he means by
koinonia in v.14 he also means in v.18 and v.20 where the same word is used.
Koinonia tends to mean a 'fellowship,' a 'sharing' or a 'participation' in something. So whatever
koinonia we experience in the Lord's Supper is the same as that which the priests experienced when they ate of the sacrifices, and that which idolators experience when they eat and drink at pagan festivals. The difference, of course, is that believers have their
koinonia with Christ, while idolators have theirs with demons.
I am aware that most Baptists today regard the Lord's Supper as simply an act of remembrance, but their 17th Century forebears understood something much deeper, and that understanding was based upon these very verses. At the Supper, there is a genuine, though spiritual, sharing in, or participation in, the blood and body of our Lord. The 1689 Confession states:
'Worthy receivers, outwardly partaking of the visible Elements in this ordinance, do then also inwardly by faith, really and indeed, yet not carnally and corporally, but spiritually receive, and feed upon Christ crucified [1 Corinthians 1:16; 11:23-26]
and all the benefits of His death; the Body and Blood of Christ, being then not corporally or carnally, but spiritually present to the faith of Believers, in that Ordinance, as the Elements themselves are to their outward senses.'
In the same way, therefore, those Corinthian Christians who visited pagan temples were in some way feeding upon demons, and the results, as illustrated in 1 Corinthians 11:17-22 (everyone, please read the passage) were disunity and disrespect for the Lord's table. The ordinance had become almost indistinguishable from a pagan festival - drunkenness, gorging, and the poor being neglected. No wonder Paul tells them,
'Therefore, when you come together in one place, it is not to eat the Lord's Supper' (v.20).