If one believes the cross of Christ is sufficient then one believes that Christ payed for all our sins ..every one of them...
When one believes his works add to their salvation or are needed to enter into the presence of God..they have an incomplete sacrifice by Christ.. He almost did it..but not quite...
SO we teach and believe. This proposition indicates only that you don't understand the teaching, since the perfect and perfectly sufficient sacrifice of Christ does not contradict it. Purgation is a gift, made available by the sacrifice of Christ and used by Him to perfect holy and free saints -- though many have died with no need of further purgation.