To be pure of heart in the biblical sense means, above all, to have our eyes focused straight on God, or to be single-minded. In other words, blessed are the single-minded, the people who take whatever happens in their lives and focus it all toward God. They are single-mindedly concerned about worshipping God in every possible way they can.
“Prayer purifies us, reading instructs us. Both are good when both are possible. Otherwise, prayer is better than reading.”
— St. Isidore of Seville