He is reflecting on how the New Testament Canon is so diverse as it contains so many very different expressions of Christianity.
"Conservatives" who want to draw firm lines of doctrine and practice out from the center in accordance with their particular tradition's interpretation of the New Testament, and "liberals" who want to sit loose to all but the central core, must both learn to accept the other as equally "in Christ", must learn to respect the other's faith and life as valid expressions of Christianity, must learn to welcome the other's attitude and style as maintaining the living diversity of the faith.
The conservative must not condemn the liberal simply because the latter does not conform to the former's particular canon within the canon. And liberals must not despise conservatives simply because the later tend to count some non-essentials among their own personal essentials (cf. Rom 14:3)"
Romans 14:3 - Good News Translation (GNT)