Unfortunately the gospel is not always pure. This seems acceptable to Paul when he says:
Phil 1:15 It is true that some preach Christ out of envy and rivalry, but others out of goodwill. 16 The latter do so out of love, knowing that I am put here for the defense of the gospel. 17 The former preach Christ out of selfish ambition, not sincerely, supposing that they can stir up trouble for me while I am in chains. 18 But what does it matter? The important thing is that in every way, whether from false motives or true, Christ is preached. And because of this I rejoice.
Also:
II Cor 2: 17 Unlike so many, we do not peddle the word of God for profit. On the contrary, in Christ we speak before God with sincerity, as those sent from God.
So it seems that though many Christians do not preach out out of good motives, the gospel still works. I likely do not answer the question directly but Satan just takes every opportunity to lie, twist and deceive as many as possible.