"Keep your spiritual fervor" literally means "keep being boiling". A follower of Jesus should be "boiling" with enthusiasm in living for and serving Jesus, and pursuing those things He loves.
But not a self-created boiling. It must be conceived and driven by the Holy Spirit. This is a by-product of spending time in scripture and prayer and living a life of obedience.
Bottom line? God wants us to be PUMPED about serving Him, seeking the lost, and building the church.