Why would Satan want to fight my ministry since it was no threat to him ?

I used to do street evangelism either holding a sign on the street corner or attached to the back of my bicycle. The one I held said "Repent" and the one on the bike said "Holiness or Hell". I also would use gospel tracts. The reason I don't feel this would be a threat to the devil is because very few people even took the warnings seriously anyway. I would feel that ineffective methods of soul winning would not pose a threat.