In "Word Focus" Darrell Grim writes:
Lange gives the meaning of first love as “that glowing, all-absorbing love to Jesus, as a personal Savior, which at the first constrained them to devoted service.” It is more than that early ecstasy that new Christians feel. John Wesley stated that the initial ecstasy of our conversion “subsides into a calm and peaceful love.” This happens even as Christ remains our first love. However, for the Ephesians that love had cooled until they were left with orthodoxy without the Spirit. As happens many times, their loss of the Spirit was followed by an emphasis on works.
Discussion: What caused the Laodicean Church to leave its first love?
Source: Christ, the Triumphant Lord: Adult Teacher's Insights, page 9.