煌めく means “to sparkle” or “to glitter,” often describing something that shines brilliantly for a brief moment. In the song, it describes a life that shines intensely despite being fleeting, giving the word a beautiful and poetic sense of precious impermanence.
King Gnu open "Prayer X" with a question most of us have asked at 2am: "一体全体何を信じればいい" (what on earth am I supposed to believe in?). It comes after "出逢いと別れを繰り返す日々の中で" (in days that keep repeating meetings and goodbyes), so the doubt grows out of something ordinary: people arrive, people leave, and nothing stays put long enough to lean on.
Then the song gets physical. We came into the world "泣き喚いていた" (screaming and crying), and life since has been about trying not to be robbed and not to "くたばる", a blunt, slangy word for dying, close to "croak". The image that sticks is "胸に刺さったナイフを抜けずにいるの" (I can't pull out the knife stuck in my chest), because the moment you do, tears burst out like spray from a wound. It is a grim picture, but the chorus answers it gently: if life is only "ひとときの煌めく命" (a life that sparkles for a moment), then maybe that brief shine is the point. Watch the word "出逢い" too: it is the poetic spelling of 出会い (meeting), used for meetings that feel like fate.