Jesus further goes on to discuss His coming and why, in verses 13 to 15. Verses 16 to 18 proclaim He died for all who believe on Him. Those who believe on Him are in obedience to His word. (There is much on obedience in the old testament as well as Romans, Corinthians, Philemon, Hebrews, Peter – a good start for study)
I’m not finding that a person can willfully sin and still enter heaven, however. Hebrews 10:1-25 gives a format for the sincere heart (verse 22) and then goes on to Christ’s judgment for those who continue in sin willfully in verses 26 to 31. It would indeed be terrifying to fall into the hands of the living God when one claims Jesus died for all their willful sins.
Wide is the path that leads to destruction and many there are who find it. There are two roads that lead to heaven, the wide path and the narrow one. Those who are on their way to heaven through Christ and obedience to God's word (the narrow) and those who think they will get there (much like the Pharisees who called Jesus a heretic and blasphemer – the wide) These on the wide path are the ones who will find themselves seated on Christ left where He will say depart from me.
We already know that the unbeliever is ‘raised up’ to God through Christ, already ‘dead in transgression” is made alive with Christ. The unbeliever walks according to the course of this world and the prince of power in the air with the spirit of disobedience. We already know to whom the unbeliever follow. (Eph 2)
The believer is called away from this life. Not as a result of our own good deeds, kind words, etc but through Christ. Because we are called through Christ; the result of this gift is walking in obedience to God’s word (good deeds, kind words, etc), which He prepared in His word beforehand as our guideline to perfection.
We are to remember that we once were separate from Christ with no hope and without God in this world but through Christ are brought near to God by the blood of Christ. (Ephesians 2)
So, yes, all sin was dealt with at the cross, just that some believe they can continue in sin without consequence but Hebrews and many other books proves differently.