Romans 3:24

and are justified freely by his grace through the redemption that came by Christ Jesus.

(v24, NIV)

Not only have all sinned, but they are justified freely by his grace. It's not saying that all sinners are justified by his grace. God doesn't justify everyone because we know that he doesn't save everyone. Instead, God is free to justify who he chooses to justify. Going back to the beginning of this section we recall that Paul is talking about a righteousness from God. This righteousness from God is to all who believe. It is true that out of all of us are sinners, but there is also no difference among those who receive a righteousness from God. Those who receive this righteousness from God are also all sinners. For these, in addition to having all sinned, they are also all justified freely by his grace. Though none even come close and they all fall short of his glory, God is the one who justifies as a free gift.