There is no union between the Prince of light and the prince of darkness

The apostles encountered those in the church who professed godliness while they were secretly cherishing iniquity. Ananias and Sapphira acted the part of deceivers, pretending to make an entire sacrifice for God, when they were covetously withholding a portion for themselves. The Spirit of truth revealed to the apostles the real character of these pretenders, and the judgments of God rid the church of this foul blot upon its purity. This signal evidence of the discerning Spirit of Christ in the church was a terror to hypocrites and evildoers. They could not long remain in connection with those who were, in habit and disposition, constant representatives of Christ; and as trials and persecution came upon His followers, those only who were willing to forsake all for the truth's sake desired to become His disciples. Thus, as long as persecution continued, the church remained comparatively pure. But as it ceased, converts were added who were less sincere and devoted, and the way was open for Satan to obtain a foothold.  

But there is no union between the Prince of light and the prince of darkness, and there can be no union between their followers. When Christians consented to unite with those who were but half converted from paganism, they entered upon a path which led further and further from the truth. Satan exulted that he had succeeded in deceiving so large a number of the followers of Christ. He then brought his power to bear more fully upon these, and inspired them to persecute those who remained true to God. None understood so well how to oppose the true Christian faith as did those who had once been its defenders; and these apostate Christians, uniting with their half-pagan companions, directed their warfare against the most essential features of the doctrines of Christ.  

It required a desperate struggle for those who would be faithful to stand firm against the deceptions and abominations which were disguised in sacerdotal garments and introduced into the church. The Bible was not accepted as the standard of faith. The doctrine of religious freedom was termed heresy, and its upholders were hated and proscribed.  

After a long and severe conflict, the faithful few decided to dissolve all union with the apostate church if she still refused to free herself from falsehood and idolatry. They saw that separation was an absolute necessity if they would obey the word of God. They dared not tolerate errors fatal to their own souls, and set an example which would imperil the faith of their children and children's children. To secure peace and unity they were ready to make any concession consistent with fidelity to God; but they felt that even peace would be too dearly purchased at the sacrifice of principle. If unity could be secured only by the compromise of truth and righteousness, then let there be difference, and even war.  

Well would it be for the church and the world if the principles that actuated those steadfast souls were revived in the hearts of God's professed people.     Great Controversy pp. 44,45