How does bad theology spread? Why do once gospel believing people, churches, organisations, and movements move away from the truth?
John Owen pondered this as he observed the spread of the "leprosy of Socinianism" (that historic collection of errors, some of which have resurfaced in contemporary evangelicalism. See this earlier post on the subject):
The vanity of the minds of men, their weariness of sound doctrine, which they will endure no longer, whatever they embrace, have given it admission, either in part or in whole, among multitudes who once professed the faith of the gospel.
And this was the case even though Owen knew that these errors had appeared, and been dealt with, on the earlier pages of church history. Owen places these errors under two heads:
1. The denial of the Trinity
Here is what he says about the second head:
But as to the latter branch of their profession...it hath diffused itself among multitudes of persons who were some time of another persuasion, and have yet engagements on them so to be.
All that unreasonable advancement of reason in matters of religion which we have amongst us; the new notions men have of the satisfaction of Christ, pretending to the acknowledgment of it, indeed destructive unto it; the noisome conception of the little use of the person of Christ in religion beyond the revelation and confirmation of the gospel; doctrines of the possibility, yea, facility of yielding acceptable obedience unto all evangelical commands without the aids of effectual grace, of the powers and incorruption of our nature, of justification by and upon our own obedience, of the suitableness of all gospel mysteries to unrenewed reason or an unsanctified mind, of regeneration as consisting only in the reformation of our lives; with a rejection of all internal real efficacy in converting grace; with the denial of any influences of grace from Jesus Christ unto the holiness of truth.
And many other opinions wherewith men even pride themselves, to the contempt of the doctrine received and established in the reformed churches of old,--are borrowed out of the storehouses of their imaginations, shall I say, or raked out of their dunghill.
And whither the infection may diffuse itself I know not. The resurrection of the same bodies substantially, the subsistence and acting of the soul in its separate state and condition, the eternity of hell torments, the nature of Christ's sacerdotal office as distinguished from his regal, begin to be either questioned or very faintly defended amongst many.
John Owen, Works Volume VII, The Nature and Causes of Apostasy, p. 77-8
It seems to me that in Owen's analysis the infection of Socinianism was spreading because the immune system among the churches was weak. There was a weakened view of the necessity of grace, or the comprehensive need of grace. This was true of the great objective work of Christ, his satisfaction for sin, and the great subjective work of God in regeneration.
Behind the diminishing of God's grace in the gospel emerged the inflation and reassertion of human ability.
Evangelicalism without the theology of the reformation churches will always revert to Pelagianism. Only the grace of God in the gospel can suppress and kill this tendency.