“For if we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins,  but a fearful expectation of judgment, and a fury of fire that will consume the adversaries.  Anyone who has set aside the law of Moses dies without mercy on the evidence of two or three witnesses.  How much worse punishment, do you think, will be deserved by the one who has spurned the Son of God, and has profaned the blood of the covenant by which he was sanctified, and has outraged the Spirit of grace?  For we know him who said, "Vengeance is mine; I will repay." And again, "The Lord will judge his people."  It is a fearful thing to fall into the hands of the living God. “ (Hebrews 10:26-31 (ESV)
I don’t know how one reads this and still holds to the idea of the perseverance of the saints.” The author of Hebrews is in any case not in agreement with Calvin. To go on sinning deliberately after receiving the knowledge of truth. The author is getting at falling away from the faith, as you can see by reading the rest of this little paragraph. Even when we keep the faith, we find ourselves sinning, even deliberately, which is why we need to keep being forgiven. The forgiveness of sins is our only ally in our fight against sin.
So often we fight against sin in our lives. All Christians do, for we all have sins to fight against. It can be a tough fight. Some struggle with drunkenness, porn addictions, a propensity to fornicate, gossiping, and uncharitable attitudes towards others, etc. How many of us don’t go through periods of outright hate towards our parents? It is perhaps within the sphere of family relations we have the hardest struggle, the first murder was fratricide. We see these sins tearing up our lives and we try to overcome them. Perhaps we pray to God for strength, and find ourselves back in the ditch 2 minutes after church. This becomes especially dangerous when we start measuring our faith by our ability to overcome these sins. If one is not careful one can quickly find themselves despairing of salvation and spurning the Son of God. He knows our sin. He knows who we are. He is faithful though, faithful to forgive. This is why the Christian life is not living the perfect life, but the forgiven life. We don’t use this forgiveness as an excuse to sin, but when we sin, and you will sin, we know that this does not count against us because we are forgiven. We live in forgiveness. We swim in forgiveness. It is the only way to keep the faith, for forgiveness is the faith. Forgiveness is what the Son of God came to accomplish, it is the purpose of his sanctifying love. We need it. We need it bad. So don’t cut yourself off from it!