Purity of mind is what St. Paul means. It follows a purity of heart. Many teachers, preachers and theologians make intelligent, meaningful and elegant proclamations. The Church is not the place for frauds and pretenders. To complete the trinity of attributes the blessed apostle tells Timothy it comes from sincere faith.
Sincere faith is omplete and absolute faith.
Statues and columns made of imperfect marble would have wax smeared over the cavities to disguise the hollow spaces. Real faith leaves no openings for myths, speculations, witchcraft, fantasies, or idle conjecture. Real faith means complete trust in God and His plan for the cosmos and us. Those of pure hearts and sound consciences shut out anything other than the whole truth about salvation through Jesus Christ in the True Church.