Why release a second “best-of” album in less than a decade? The Greatest Hits, the Christian vocal quartet’s 2009 song collection, pares down the track list from 2003’s Testify to Love anthology and offers a more concentrated summary of its harmony-centered sound and faith-based message. The effervescent pop of “Give it Up,” the silken R&B of “Testify to Love,” and the reverent yet danceable “Knockin’ On Heaven’s Door” capture the group’s versatility in the ‘90s under the production guidance of Charlie Peacock. As its career progressed, tunes like the arena rocker “The Glory” and the epochal ballad “Can’t Live a Day” found Avalon expanding further with Brown Bannister at the studio helm. Weathering line-up changes and shifting trends, Avalon has remained relevant with ear-catching tracks like “All” (a breezy Europop number) and “In Christ Alone” (an appealing mix of acoustic guitar and tribal rhythms). What’s remained constant is their fervent, unambiguous dedication to spreading Christ’s word, a mission reaffirmed in the newly recorded tune “Still My God.”