Why was Antestor known as sorrow metal?

Context: Antestor is a Norwegian Christian extreme metal band formed in 1990 in Jessheim. Credited for starting the northern European Christian black metal scene, Antestor is the only Christian band to have an album released by Cacophonous Records, which has also released records by bands such as Dimmu Borgir, Sigh, and Cradle of Filth. The band's only release on Cacophonous, The Return of the Black Death, proved influential for the Christian black metal movement, and has sold over 10,000 copies.

In the late 1990s they dubbed their musical style as "sorrow metal" rather than black metal because the black metal movement was publicly affiliated with Satanism in Norway. According to HM magazine, the progressive elements on the debut album, Martyrium, were possibly ahead of their time in the Norwegian extreme metal scene.

The group has gone through several line-up changes over the years, and currently Antestor consists of the founding guitarist Lars Stokstad, vocalist Ronny Hansen, bassist Erik Normann Aanonsen, guitarist Robert Bordevick, and drummer Henning Børven. The original vocalist Kjetil Molnes and drummer Svein Sander left the band around 2000, and the members of a fellow Norwegian black metal group, Vaakevandring, joined Antestor. The reputable Norwegian extreme metal drummer Jan Axel Blomberg played session drums for their 2005 album The Forsaken and Det tapte liv EP. A fourth studio album, Omen, was released in 2012.

Answer: Because black metal was associated with Satanism in Antestor's home country Norway, and they are a Christian extreme metal band.