Assurance is not based upon our good works

"Matthew 7:22-23 reads, 'Many will say to me in that day, Lord, Lord, have we not prophesied in thy name . . . ? And then will I profess unto them, I never knew you.'
This is severe, and very strange. These people against whom Jesus shall shut the door of heaven are not the secularists who openly despise religion. These are the people who attend church regularly, the ones who do the work, who boast of their success. These are the religious leaders, who talk of leading the church of Christ. But Jesus says, I never knew you. False assurance is a common thing.
Yet it is possible to have a true assurance and rejoice in a hope that will not disappoint. The Scriptures say it is possible and urge us to attain to that state. John wrote his first epistle especially to teach us this lesson." - Gordon H. Clark, What Do Presbyterians Believe?