Manowar mainly performs songs dedicated to warrior cultures (with a large focus on Norse). But Manowar also performs material with Christian overtones:
"Oh, I wish, I was in the land of cotton
Old times they are not forgotten
Look away, look away, look away Dixie land
Oh, I wish, I was in Dixie away, away
In Dixie land I'll take my stand
To live and die in Dixie
Dixie land where I was born
Early Lord, one frosty morning
Look away, look away, look away Dixie land
Glory, glory, Hallelujah
Glory, glory, Hallelujah
Glory, glory, Hallelujah
This truth is marching on
So hush little baby, don't you cry
You know your daddy's bound to die
All my trials Lord, will soon be over
Glory, glory, Hallelujah
This truth is marching on
This truth is marching on"
Should we judge songs by their association? Or by their content? Even if we are to judge by association and point a finger at Manowar, Sabaton has a wide range of songs about Christian history including battles and righteous crusades. Kingdom Come was written in a way that might speak differently to different people, but it very specifically meshes well with Christian imagery and that is exactly what Sabaton ran with.
Another example of this kind of "conversion by association" would be Trent Reznor's Hurt. Hurt was likely intended as a song about nihilism but was later covered by Johnny Cash from a Christian perspective (changing a word and relating the lyrics to Christian imagery). Should we be critical of Johnny Cash's Hurt because of other songs Trent Reznor had written?