Hebrews 10:11-14 (ESV)
And every priest stands daily at his service, offering repeatedly the same sacrifices, which can never take away sins.  But when Christ had offered for all time a single sacrifice for sins, he sat down at the right hand of God,  waiting from that time until his enemies should be made a footstool for his feet.  For by a single offering he has perfected for all time those who are being sanctified.
“For by a single offering he has perfected for all time those who are being sanctified.” So it is we are sanctified, and are constantly being sanctified, as often as we are being forgiven our sins. It is an odd paradox, yet what do you expect when dealing with God? The one thing here that does become appearant is that our sanctification, ongoing yet completed, is in the one sacrifice of Christ.