Penal Substitutionary Atonement
When you encounter professing Christians who attempt to deny penal substitutionary atonement, I would encourage you to make sure to point them to Luke 22:37 (among many other passages). Here Jesus is speaking just prior to Jesus' crucifixion, where He defines the importance and meaning of His going to the cross:
"For I tell you that this Scripture MUST be FULFULLED in ME: ‘And he was numbered with the transgressors.’ For what is written about me has its FULFILLMENT.” Luke 22:37 (emphasis mine)
Jesus is here pointing back to a quote in Isaiah 53 claiming that it is speaking of Him and that it finds its purpose and end in Him. Anyone who is familiar with that chapter of the Bible knows that it explicitly predicts the suffering servant's substitutionary atoning work for His people.