Wesley's false gospel is based on the strides made in sanctification

"John Wesley believed in justification by faith. His 'long suit,' however, was sanctification. He had been deeply influenced by Moravian Pietism and certain of the great Roman Catholic mystics. But Wesley's emphasis on sanctification was the weakness of the Methodist movement. Along with justification by the blood of Christ, Wesley emphasized the renewing power of the Holy Spirit in conforming lives to true obedience to the law of God. Apart from sanctified obedience to the law of God, Wesley declared that no soul would retain the blessing of justification. 
Wesley developed a doctrine of entire sanctification, known also as the 'second blessing' or 'Methodist perfection.' He proposed that after justification and a process of sanctification, the believer could receive by faith a sudden second blessing which would completely purge the soul from inbfred sin, enabling the fully sanctified to feel nothing but perfect love. He called his experience 'a still higher salvation,' 'immensely greater than that wrought when he was justified' (Plain Account, p. 7). Wesley and his preachers urged their hearers to seek this second blessing of perfection with all diligence. They did, and gave proof of it in lives of earnest (and sometimes frantic) piety.
With Paul and Luther, justification by faith was the whole truth of the Gospel. But in Wesleyanism, the centrality and all-sufficiency of justification tended to be lost by being subordinated to sanctification." - John Robbins, Found in the Appendix in The Holy Spirit, Pg. 104