The next church Jesus speaks to in Revelation is the church at Pergamum. It was a large city, home to the worship of many different gods (and emperors), including Aesculapius, the god of healing. It was the city, Jesus said, where Satan has his throne. The church seems to have withstood an intense time of persecution, including the death of Antipas. But Jesus judges them because false teachers in the church are leading people astray. These teachers lured people into dangerous compromises and accommodations to the culture around them. They said it was ok to participate in pagan worship (meat sacrificed to idols) and also engage in sexual immorality. Jesus said they must repent and that those who were victorious (persevering in faith) would be rewarded.
For the Church today, false teaching is no less an issue. It is very tempting to teach people that compromising their faith and accommodating themselves to the surrounding culture is ok. Even today, the Church must guard against a real enemy (Satan), we must guard against false teaching, and we must guard against the temptation to conform to the world.