This is how millions understand it, and most of the world will be in happy agreement with it, but it is not the gospel.

For one it leaves room for boasting and in fact results in boasting before the Lord. You may say, "No this isn't boasting before the Lord", which will only show that you don't understand what boasting means. When you stand before God and you argue your case you made to me, saying salvation is ultimately up to man, that salvation is not conditioned on the work of Christ, and that His people aren't justified by His blood, then you will condemn yourself by your own words. You must repent of all these thoughts and you must repent of your wicked way and believe God's way. That salvation is all of God, that salvation is conditioned on the work of Christ.

Isaiah says "by His wounds we are healed." If He was slain for everyone, then everyone would be healed. But He was slain for the sins of His people.