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)