If you no longer believe/ trust in Christ you no longer have the blood of Christ to cover you sins.
Your sinning from that point forward is held to your account, not Christ's anymore.
Christ does not pay the sin debt of people who do not believe/ trust in Him to do that.
The condition for having your sins forgiven is that you believe/ trust in Him.
You don't work to have your sins forgiven, you trust in the ministry of Christ in order to have your sins forgiven.
26If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, 27but only a fearful expectation of judgment" - Hebrews 10:26
(Note: This is talking about the person who willfully sins in unbelief and a rejection of Christ. It's not talking about the struggling saint moving on to maturity through their continued believing. The blood of Christ covers that person. They still believe.)