Here a few of my favorite quotes from Spurgeon:
"The day will come when those who think they can repair a house which has no foundations will see the wisdom in quitting it altogether. All along we have said that to come out from association with questionable doctrines is the only possible solution of a difficulty which, however it may be denied, is not to be trifled with by those who are conscious of its terrible reality."—The Sword and the Trowel, July 1889.
"Complicity with error will take from the best of men the power to enter any successful protest against it. If any body of believers had errorists among them, but were resolute to deal with them in the name of the Lord, all might come right; but confederacies founded upon the principle that all may enter, whatever views they hold, are based upon disloyalty to the truth of God."—The Sword and the Trowel, October 1888.
"My course has been of another kind. As soon as I saw, or thought I saw, that error had become firmly established, I did not deliberate, but quitted the body at once. Since then my one counsel has been, 'Come ye out from among them.' If I have rejoiced in the loyalty to Christ's truth which has been shown in other courses of action, yet I have felt that no protest could be equal to that of distinct separation from known evil."—The Sword and the Trowel, December 1888.
"Constantly we hear of proposals for union, and truly these are welcome where mere technical matters divide true Christians; but what is the use of pretending to create union where there can be none? There is another matter which needs to be thought of as well as union, and that is TRUTH. To part with truth to show charity is to betray our Lord with a kiss. Between those who believe in the eternal verities and those who constantly cast doubt on them there can be no union. One cried of old, 'Is it peace?' And the answer was a sharp and true one. We render it thus—'What hast thou to do with peace while departures from the truth of God are so many?' The first question is, 'Are we one in Christ?' And are we obedient to the truth revealed in the Scriptures? If so, union will necessarily follow: but if not, it is vain to clamour for a confederacy which would only be an agreement to aid and abet each other's errors."—The Sword and the Trowel, February 1887.
"For Christians to be linked in association with ministries who do not preach the gospel of Christ is to incur moral guilt. A Union which can continue irrespective of whether its member churches belong to a common faith is not fulfilling any scriptural function. The preservation of a denominational association when it is powerless to discipline heretics cannot be justified on the grounds of the preservation of 'Christian unity'. . .It is error which breaks the unity of churches, and to remain in a denominational alignment which condones error is to support schism."—The Forgotten Spurgeon, pp. 164-165.