Paul prays that the growth of love in knowledge and discernment may result in Christians better approving what is excellent—that which really matters, that which is essential. Then the believers will be pure and blameless for the day of Christ.
Pure meaning inwardly pure. Blameless: that there is nothing to fault about them.
10 so that you may approve what is excellent, and so be pure and blameless for the day of Christ,