It is difficult for modern man to establish the relationship between “symbol” and “devil”, but that relationship exists.
σύμβολον, symbolon, symbol, as dictionary says, means contract, trade agreement, mark, badge, sign, password, emblem, symbol, logo, omen, auspice, convention, treaty. It relates to συμβάλλω, symballo, take, gather, gather, recruit, launch, compare, interpret, agree … συμ- or συν- , sin-, is a Greek preposition or preverb meaning "with" and therefore implies an idea of community, of union, of meeting.
“Symbol” makes regard to a practice of protection of hospitality or completion of contracts and agreements; it consists on break a piece of pottery, metal and other materials so that each of the two parties, the traveler and the host, the seller and the buyer, stayed with one of them; on a future occasion the meeting of the two sides in one piece was the evidence of ancient hospitality established or agreement made. Then, in the legal world, the symbol was a contract between private individuals with certain legal guarantees.
διάβολος, diabolos, devil, means slanderer, detractor. It relates δια-βάλλω, dia-ballo, throwing, rift, alienate, accuse, slander, denigrate, cheat … διά, dia-, is exactly a preposition or adverb meaning separately, in part, through .. ; it is, the opposite of συν , sin-, with.
The term is related to διὰβολή, diabole, that means disagreement, quarrel, false accusation, slander.
Actually the word diabole originally referred to the position that in judicial proceedings, in court, kept who doubted the claim of the other party, which is called diabolos , διάβολος. From this rhetorical sense step to mean "slanderer" and diabole "slander". In the Christian context the devil was who introduces division and estrangement from the truth of God and therefore was the enemy of God.
So etymologically "symbol" is the opposite of "devil": if the "symbols" join together, the "devils" separate.
Demons (from Lat. daemonium, and this from Gr. δαιμόνιον , daimonion, and δαιμόν, daimon ) deserves a separate treatment which in turn take out.