Christ is not only the propitiation for the sins of Christians:
1 John 2:1-2 (ESV) My little children, I am writing these things to you so that you may not sin. But if anyone does sin, we have an advocate with the Father, Jesus Christ the righteous. He is the propitiation for our sins, and not for ours only but also for the sins of the whole world.1
Even apostates and unbelievers were paid for:
2 Peter 2:1 But there were also false prophets among the people, even as there shall be among you lying teachers, who shall bring in sects of perdition, and deny the Lord who bought2 them: bringing upon themselves swift destruction.
Hebrews 10:19-31 Therefore, brothers, since we have confidence to enter the holy places by the blood of Jesus, by the new and living way that he opened for us through the curtain, that is, through his flesh, and since we have a great priest over the house of God, let us draw near with a true heart in full assurance of faith, with our hearts sprinkled clean from an evil conscience and our bodies washed with pure water,3 Let us hold fast the confession of our hope without wavering, for he who promised is faithful. And let us consider how to stir up one another to love and good works, not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near. 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 trampled underfoot 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.
Clearly, we can do worse than disobeying an inferior Covenant, by disobeying the more magnificent New. Even if we were sanctified by the blood of that Covenant.
The exact same goes for the following:
2 Peter 2:20-22 For if, after they have escaped the defilements of the world through the knowledge of our Lord and Savior Jesus Christ, they are again entangled in them and overcome, the last state has become worse for them than the first. For it would have been better for them never to have known the way of righteousness than after knowing it to turn back from the holy commandment delivered to them. What the true proverb says has happened to them: “The dog returns to its own vomit, and the sow, after washing herself, returns to wallow in the mire.”
The reason those who were washed by the blood of Jesus being lost disproves limited atonement is for the simple reason that if He hadn't atoned for them, no alternative redemption was available for them from the defilement of the world.
See also Heb 10:36-38b. God's righteous one (only possible if redeemed by Christ) can shrink back from his calling and have no part with God. cf. Rev 2:5.
But the simplest of all: God wants all to be saved, so our opting into salvation isi what stands in the way of going to heaven, not what Christ has done for us:
1 Timothy 2:3-4 This is good, and it is pleasing in the sight of God our Savior, who desires all people to be saved and to come to the knowledge of the truth.
cf. Ezekiel 33:11.
1 i.e. the antithesis of 'Limited Atonement'
2 cf. 1 Cor 6:20
3 This person was baptized into Christ, and has "put on Christ." Gal 3:27.Rom 6:4; Col 2:12. Eph 5:25-27; 1 Pet 3:21 (is "[saved]"); Ezek 36:24-25 etc.