This is a good quote from a denominational scholar in talking about the need to restore the one NT church and the unity for which Jesus prayed (Jn 17:20-24). C.E.B. Cranfield is a well-known Reformed theologian and was Emeritus Professor of Theology at the University of Durham, England.
The Sinfulness of Denominational Divisions
The New Testament never treats this agreeing in Christ as an unnecessary though highly desirable spiritual luxury, but as something essential to the true being of the Church. Divisions, whether disagreements between individual members of the existence of factions and parties and – how much more! – our present-day denominations, constitute a calling in question of the Gospel itself and a sign that those who are involved are carnal.
The more seriously we take the New Testament, the more urgent and painful becomes our sense of the sinfulness of the divisions, and the more earnest our prayers and strivings after the peace and unity of the Church on earth….[I]t is to be a unity in which powerful tensions are held together by an over-mastering loyalty, and strong antipathies…are overcome in common worship and common obedience.
Such unity will only come when Christians are humble and bold enough to lay hold on the unity already given in Christ and to take it more seriously than their own self-importance and sin, and to make of these deep differences of doctrine, which originate in our imperfect understanding of the Gospel and which we dare not belittle, not an excuse for letting go of one another or staying apart, but rather an incentive for a more earnest seeking in fellowship together to hear and obey the voice of Christ.
– quoted by William Barclay, Letters of James and Peter, 225-226
–W. Frank Walton