“Repent and believe” is what Jesus preached.
My late father, a pastor, used to tell me it is one thing to have great exegesis skills, the real challenge is in how to apply the word. The message given to the people at that time meant something particular to them. Generalising it to include ourselves then applying it in preaching is the great challenge. If you have been doubting Christ even though His word is being presented accurately and clearly to you, “repent and believe” is obviously applicable. When it gets tricky is when people have been mentally assenting for years to the preaching they hear. Repenting is less obvious. Believing is less obvious. Yet we each should test ourselves inwardly as to whether we are truly accepting the word planted in us which can save us. God sees our hearts. If God sees our real faith then God will grant our hearts the anointing of the Holy Spirit.
