Jesus describes himself as the one who has the seven spirits of God and the seven stars. The seven spirits of God are mentioned in the doxology in Revelation 1:4-5 as being, along with the Father and the Son, the source of divine grace and peace. Since only a divine person could be so described, and since the Father and the Son are also mentioned, the seven spirits of God must be a way of describing the Holy Spirit. The number ‘seven’ is often used to describe perfection.
Jesus received the Holy Spirit in a special manner when he ascended to heaven. He had described this reception as being connected to a promise made by the Father to give him the Spirit in this new way. Jesus also stated that the coming of the Spirit from him to his disciples would mean that they would experience incredible power as they declared the gospel and commenced the New Testament church. We see what this looked like in Acts 2.
It is clear from the description of the church in Sardis here that they were not experiencing the power of the Spirit. Instead of being marked by spiritual life, they were the opposite. What they needed was to experience anew the refreshing presence of the Spirit in their meetings and activities. So when Jesus describes himself as having the Spirit, he is making an incredible offer to the church there. He is reminding them that they can have the Spirit in power once again.
I suspect that many churches go through a Sardis experience, some for longer than others. The church in Sardis seemed content to stay there, and other churches had not realised there was a problem. Yet it is not difficult to work out if the Spirit is present in power. Are there conversions? Is the gospel drunk as living water? Is brotherly love obvious? If not, Jesus has the remedy – the presence of the powerful Spirit of God.
The reference to Jesus having the seven stars (the angels of each of the seven churches) is probably another reference to his power and authority. Maybe the church in Sardis would not like to hear what Jesus had to say to them through his messenger. They needed to be reminded that the messenger had Jesus behind him as they listened to what was in the letter. Maybe we need to hear it as well.