Phillip Johnson says that "the evangelical political right has historically -- not just theoretically -- fostered an ecumenical drift." I'm not quite sure whether by 'ecumenical drift' Johnson means Christians working with non-Christians in various endeavors, or whether he means Christians accepting non-Christians as their brethren. I'm assuming he means the latter. If so, let's concede this historical claim for the sake of argument. The question is as to its significance. Johnson says that this fact means that concern over political activism leading to ecumenism is not "a groundless or superstitious fear". That seems right; history should always give us pause. But should we give this historical claim any larger, more sweeping significance? In particular, does it mean that it is immoral or unwise for Christians (in large numbers or small) to cooperate with non-Christians of various stripes in political activism?
I submit that to think so is to fall afoul, if not of the genetic fallacy, then at least of the slippery slope fallacy. A good treatment of the latter is in John Frame, Doctrine of the Knowledge of God (P&R, 1987). Frame says on pp. 274-275:
"A true and valid reductio must be distinguished from its fallacious imitators, one of which is the 'slippery slope' argument. A slippery slope argument goes like this. 'If you take position A, you run the risk of taking position B; position B is wrong, therefore A is also wrong.' Thus it is sometimes said that once one abandons belief in a pretribulational rapture, he runs the risk of denying the bodily return of Christ altogether, thus opening himself up to a thoroughgoing liberalism. Or it is sometimes argued that if one accepts the textual criticism of Westcott and Hort, he runs the risk of denying biblical authority altogether. Thus the slippery slope argument appeals to fear -- to our fear of taking undue risks and to our fear of being linked with people (such as liberals), disapproved of in our circles, lest we incur guilt by association.
Often slippery slope arguments are buttressed by historical examples. Such-and-such a theologian began by denying, say, total abstinence from alcoholic beverages, and five years later he abandoned the Christian faith. Or such-and-such a denomination rejected the exclusive use of Psalms as hymns in worship, and twenty-five years later it capitulated to liberalism. On the use of such historical references in theological arguments, see chapter 9. In general, they prove nothing. Usually, they do not rest on a sufficient statistical sample to establish even probable conclusions. And they ignore the complexities of historical causation. A denomination becomes liberal for many reasons, never just one. On the one hand, it may well be that rejection of exclusive Psalmody is in some cases at least a symptom of advancing liberalism. (I say that as an opponent of exclusive Psalmody, who nevertheless recognizes that people sometimes reject exclusive Psalmody for very bad reasons.) On the other hand, the denomination may be rejecting exclusive Psalmody for good reasons. This development may be quite independent of any trend toward liberalism, or it may bear a paradoxical relation to that trend. For example, the liberal trend may, for a time, help the church to break free of unbiblical traditions -- God's bringing a good result out of an overall evil development. (It could be argued that the development toward liberalism in the Presbyterian Church U.S., for example, enabled that denomination to take a strong stand against dispensationalism, a stand that to many nonliberals was a good thing.) Thus not very much can be deduced from historical examples. They ought to make us think twice about what we are doing. They suggest possibilities, but they are never normative in themselves."