As the Church, What is Our FIRST Love? #154 #Revelation

In the book of Revelation, Jesus writes a letter to seven different churches. In His first letter, to the church at Ephesus, He tells them that - even though they have been faithful in many ways - they have left their FIRST love...and because of this, they are on the verge of ceasing to be a church. What is the first love that Jesus is referring to here, and why is abandoning it so serious?