2:4 `But I have this against you, that you have left your first love.
BUT, I have this against you, that YOU HAVE LEFT YOUR FIRST LOVE.
It is not clear what was meant by "FIRST LOVE". It may have referred to the original love the members at the church at Ephesus had for each other. Organization and "church" activities probably had replaced the "first love" as the church had grown.
The apostle Paul retained his first love. In fact, it became stronger, broader, richer, deeper, and truer - Philippians 3:7-11.
"FIRST LOVE" is
* exclusive - It excludes the world and the flesh and resists the devil.
* makes Jesus Christ preeminent - Keep seeking things above, where Christ is. Colossians 3:1-4
* very active - It seeks to please the Lord Jesus Christ by keeping His Words. John 14:21-24
If you have "lost your first love", He, Jesus Christ, has "somewhat against you." If you never have had that "first love", now is time to get that first love.