He was referring to Christian freedom. Those who appreciated that freedom slaved for one another out of love.
وَٱلَّذِينَ كَانُوا يُقَدِّرُونَ هذِهِ ٱلْحُرِّيَّةَ خَدَمُوا بَعْضُهُمْ بَعْضًا كَعَبِيدٍ بِدَافِعِ ٱلْمَحَبَّةِ .‏