Nevertheless the solid foundation of God stands, having this seal: "The Lord knows those who are His," and, "Let everyone who names the name of Christ depart from iniquity."
2 Timothy 2:19

Paul had been writing about those who had strayed from the faith. That should be a scary subject since some who fall away seem like the least likely to do so. Paul offers a word of encouragement and one of a warning. Both come from Numbers 16, verses 5 and 26 respectively.

The context in Numbers was the rebellion of Korah, which cost many their lives. Nevertheless, God did safely deliver His people into the Promised Land.

Paul’s encouragement was that while some fall away, “the Lord knows those who are His.” The Good Shepherd knows His sheep and He will bring them all safely home. Be encouraged that if you belong to Christ, your salvation is secure.

Paul’s warning, "Let everyone who names the name of Christ depart from iniquity," is a sober reminder that assurance is ours only as we walk pervasively in obedience (though none of us does so perfectly).

God will save His people. His people are marked by the pursuit of holiness.