The answer to this question is actually in 1 John 2:15 but before dealing with that let me set out what we are discussing in 1 John 4:8
The one not loving has not known God, because God is love.
Note that here we have both the noun and verb cognate forms of love:
- ἀγαπάω (agapaó) - verb meaning "I love, wish well to, take pleasure in, long for; denotes the love of reason, esteem" (NAS)
- ἀγάπη (agapé) - noun meaning "love, benevolence, good will, esteem" (NAS)
In 2 Tim 4:10 ("Demas loved the world") we have have the verb form of the word love. Now, let me quote 1 John 2:15 -
Do not love the world or anything in the world. If anyone loves the
world, the love of the Father is not in him.
Note the distinction John makes between his remarks in 1 John 2:15 vs 4:8 - do NOT love the world but love God and the each other as God has loved you. Note the instruction we have later in the same chapter of 1 John 4:
15 If anyone confesses that Jesus is the Son of God, God abides in him, and he in God. 16 And we have come to know and believe the
love that God has for us. God is love; whoever abides in love abides
in God, and God in him. 17 In this way, love has been perfected
among us, so that we may have confidence on the day of judgment; for
in this world we are just like Him.
18 There is no fear in love, but perfect love drives out fear, because fear involves punishment. The one who fears has not been
perfected in love. 19 We love because He first loved us.
This is the complete opposite of what we find in 1 John 2:15 vs 1 John 4, namely
- do not love the world
- love God and the each other because love comes from God. We love because God first loved us.
On this basis, Demas as per 2 Tim 4:10, did exactly what John's instruction told him NOT to do - loved the world instead of loving God because God loved him.