The Church is the bride of Christ. As I have already covered: the Church universal is represented by the church local. So for someone to say that they love Jesus but not his church, is to say that they love him but hate his bride. Jesus builds an analogy of his bride being salt and light (Matt. 5:13-17). She is supposed to stand out and be different. In Jesus' day salt was very useful not only for adding flavor to food but also for preserving food. It was a daily essential. The church is made up of people who are just as essential.
Scripture offers several more metaphors for the Church. Those who make up the church are described as branches on a vine (John 15:5), a field of crops (1 Cor. 3:6-9), an olive tree (Rom. 11:17-24), a building (1 Cor. 3:9), a harvest (Matt. 13:1-30; John 4:35), a holy priesthood (1 Peter 2:5), a pillar of truth (1 Tim3:15), and a new temple (1 Cor 3:16-17). All of these metaphors can be used to explain why being a part of the local church is essential for a believer, but let's look at one final analogy that is used in several different places in scripture. In 1 Corinthians 12 Paul explains that the church is the body of Christ which is composed of different parts. We have different roles to play in that body. So for someone to claim to be under the headship of Christ (Eph 1:22-23), but to not be an active part of his body is nonsensical. If Christ shed his blood for you and has redeemed you from your sins, he has a purpose and plan for you as a part of his body. To follow Christ means to submit to him as the head, and the head wants the body together. A hand that is doing its own thing apart from the rest of the body is useless.
"For we are his workmanship, created in Christ Jesus for good works, which God prepared beforehand so that we would walk in them." (Eph 2:10)