It’s rather clear that they are stepping on the Name of our Lord Jesus Christ. It also appears that that there are flames coming from the Christogram.
I do think that you are correct. The four horsemen heroes of the Reformation are trampling on Jesus and condemning Him to the flames of hell. The Reformers speak of Catholic error; here they are not only placed above Jesus, they are stepping on Him and sacrificing Him to hell forevermore. What pointed and sacriligious graven imagery. I hope that the Reformed will all rise up in anger and call for the destruction of this blasphemous graven imagery.