The following quote comes from Iain Murray’s book Evangelicalism Divided (on page 291 if you must know):
The ecumenical call [in the mid-20th century] was not for truth and salt; it was supremely for oneness: the greater the unity of ‘the Church’, it was confidently asserted, the stronger would be the impression made upon the world; and to attain that end churches should be inclusive and tolerant. But it has never been by putting unity first that the church has changed the world. At no point in church history has the mere unity of numbers ever made a transforming spiritual impression upon others. On the contrary, it was the very period known as ‘the dark ages’ that the Papacy could claim her greatest unity in western Europe.
So if we would have true unity, we must have theology. We are to share, profess and enjoy unity with other believers, even those who do not share certain “lesser” doctrines. This is not to imply that any doctrine is unimportant, but simply that some are more important than others. J.C. Ryle wisely observed that believers should “keep the walls of separation as low as possible, and shake hands over them as often as you can.” But there are times when we must reject supposed unity because of the higher importance of truth and sound doctrine. To repeat Murray’s words, “it has never been by putting unity first that the church has changed the world.” Nor will it ever be.