Christians have no obligation to seek the purest church they can find and stay there, and then leave it if an even purer church comes to their attention. Rather, they should find a true church in which they can have effective ministry and in which they will experience Christian growth as well, and then should stay there and minister, continually working for the purity of that church.
Systematic Theology 874-875
[N]o one is permitted to spurn its [the church’s] authority, flout its warnings, resist its counsels, or make light of its chastisements — much less to desert it and break its unity. For the Lord esteems the communion of his church so highly that he counts as a traitor and apostate from Christianity anyone who arrogantly leaves any Christian society, provided it cherishes the true ministry of Word and sacraments. He so esteems the authority of the church that when it is violated he believes his own diminished.
Both quotes from Sojourners and Strangers by Gregg Allison, pp. 166-7.