In his booklet Stand for the Truth, Rev. Dr. Peter Masters of the Metropolitan Tabernacle informs us of the dangers of not practicing secondary separation. In his own words:
... When professing evangelicals (especially leaders) disobey the command of God to maintain a distinctive testimony, unassociated with false teachers, they participate in the evil deeds of the latter in at least four different ways:
(1) Non-separators deal a terrible blow to the exclusive nature of the message of the Gospel. When evangelicals are seen to accommodate the viewpoint of non-evangelicals and to accept their claims to spiritual life, the clear teaching of the Word is undermined. The line between Truth and error becomes blurred, and also the distinction between saved and unsaved. Believers who look on are liable to stop thinking of 'conversion' in precise, evangelical terms.
(2) Non-separators help the devil to achieve one of his main objectives — to cause such confusion that the world no longer sees a distinctive, biblical Christian community standing clearly apart from Catholic and liberal error. Non-separating evangelicals communicate to the world the idea that all so-called Christian churches are the same. Nothing is more crippling to the true testimony of evangelicalism. In the nineteenth century even unsaved people knew the Protestant arguments against Catholic dogma. But today (through compromise) evangelicals have long since surrendered any distinctive place in the general knowledge of the public.
(3) Non-separators lower the guard of the people of God, exposing them to infiltration by false believers and false doctrine. Once the people follow the example of their non-separating ministers, learning to tolerate and accept 'other views', then the way is prepared for a major doctrinal collapse. Non-evangelicals could never penetrate evangelical churches without 'inside help'. They need a 'pass' into the household of God, which only an evangelical 'collaborator' can give them, as he extends to them credibility, recognition and opportunity
(4) Non-separators encourage false teachers in their infidelity and sin, and so strengthen in their work. The rise of theological liberalism in the denominations, and its take-over of them, was funded almost entirely by evangelicals. When these opponents of the Gospel first infiltrated colleges and publishing institutes, non-separating evangelicals continued to pay the bills and support them. Almost all the present-day liberal theological colleges and churches in the historic denominations were originally built by the blood, sweat and toil of evangelicals. Then, a subsequent generation of non-separating evangelicals gave them away! This continues today in denominations where evangelicals give their money to support hoe and overseas missions mainly staffed and run by non-evangelicals. Non only do non-separating evangelicals support false teachers at a practical level, but they also seal them in their spiritual delusion by failing to challenge them about their lost spiritual condition.
There are the tragic consequences of disobedience to God. Satan rejoices, the Holy Spirit is grieved, and the cause of Christ is seriously wounded. If the non-separators are ministers, their flocks see a major area of biblical teaching set aside by their spiritual leaders, and thus the authority of the Bible is further undermined. The words of 2 John 11 condemn compromise. They indicate that the person who places himself on the side of the enemy must be regarded as guilty of a grievous sin against the Lord and His work. The non-separator makes himself a sharer of the damage done by the false teacher. Without doubt there is guilt by association. (pp. 16-18)
It is ridiculous for some to use the example of Charles Simeon in his interaction with John Wesley, in the case of John Piper. Wesley was an evangelical who affirmed and practiced evangelical doctrine. Warren on the other hand professes to be an evangelical but does not practice any of it. His ministry is a mishmash of Pelagianism/Market-drivenness, Pragmatism, Moralism, "Doctrine as mere knowledge" doctrinal minimalism, and the practice of kicking out resistors who oppose his unbiblical vision. The Gospel is mangled beyond recognition, the Church is treated like a corporation, members as slaves of said corporation etc. There is simply no comparison possible.