Don't love the world nor the things in the world. If anyone loves the world, love for the Father isn't in him. Because all that is in the world, the lust of the flesh and the lust of the eyes and the vainglory of life, is not of the Father but is of the world was written by John the apostle, like all his writings, apparently near the end of his life and in the Gentile world. That use of the word "world" doesn't sound like it's talking about Judaism (like Paul might be in Gal 1:4, the "present evil age" maybe of Judaizing and law-keeping, as indicated by the different gospel, which is not another gospel, only there are some who trouble you and desire to pervert the gospel of Christ ). Rather: the secular, ordinary world catering to people's needs and wants.
"Demas" looks like a Gentile name, Paul's coworker, who very near the end of Paul's life, left him for "the world." 2 Timothy touches false teaching, but not Judaizing that I can see. I suppose Demas may have been influenced by it, but it seems Paul would have been more specific that that was Demas' problem.
"Having loved" in the past tense sounds like it's pointing to the cause that the brother left the apostle. Effectively equaling loving the world. I think "apostasy" means "falling away" (from where you were standing). So Demas fell away at that point. Whether he remained so, I don't know. I wouldn't apply the word "lost" because it seems that all its uses in the NT are about folks who need to encounter and receive Christ, whereas Demas already must have. Birth (Jn 1:12-13; 3) from God, just like physical birth, by definition cannot be lost. Once one's born of God, one's always born of Him (Heb 12). There are serious consequences though for wasting our time as believers, both in this age and the next (Mt 24--25). We (believers) will all stand before the judgment seat of God [who is Christ]...So then each one of us will give an account concerning himself to God Rm 14:10, 12; 2 Cor 5:10; 1 Jn 2:28; Philip 2:12; 3:12; Rv 11:18; 22:12; Mt 7:21-23; etc.