I don’t see how in the world anyone could read this passage and not understand that salvation is a gift of God’s grace and we receive that gift by faith alone. How much clearer does he need to be? Anytime one adds extra stuff to be done to either obtain salvation or keep salvation, it nullifies grace. Grace MEANS undeserved, unmerited, unearned.
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Pride in their own deeds. Desire to boast.
Jesus is going to be telling them ‘I never knew you.’
Scripture tells us over and over that NO ONE will be able to boast before God. Those who fall for an accursed and perverted gospel will be condemned because they did not receive the gift of God through faith.