Seeing that God exists outside of time and sees the past, present and future all at once, then when Jesus died on the cross, He took the penalty for all the sin that God could see - past, present and future. So that when we received Christ as Saviour, all our sin was placed under the blood of Christ, past, present and future. This is because God not only saw our past sins, but He also sees every sin we will ever commit. It is a fact that even the most righteous believer will fall several times a day, and if there is not the assurance that God has forgiven and cleansed us from all sin, including our future ones, how could we ever have full assurance of salvation?
Of course, some contentious hockey puck will come along and counter this with the old chestnut that believers can go on wilfully engaging in the sinful works of the flesh and still expect to have assurance of salvation, but Paul puts this one to bed by saying, "Where sin does abound, grace does much more abound; so, shall we sin [ie: wilfully continue in the works of the flesh] that grace may abound? God forbid! How can we being dead to sin [wilfully] continue in it?" (The bits in the square brackets is my definition of the type of sin I am talking about.).
The teaching that we have to have Jesus crucified and sacrificed afresh every time we sin is that satanic, pagan Roman Catholic ceremony of the Mass which has the belief that the wafer is the real body of Christ and the wine is the real blood of Christ. So the priest actually eats Christ's body! And deluded souls pray to the box which contains the wafers, believing that the real body of Christ is in it. The whole thing is sheer pagan idolatry and an abomination to God!
Jesus was crucified once, and the Scripture says that He will never die again (Romans 6:9), making the continued sacrifice of the Mass a fraud, and the Jesus who dies every time the Mass is performed one of the imagination.
This therefore means, that the one sacrifice of Christ on the cross, has canceled every sin for the converted believer, past, present, and future.