Matthew 5:37 — “Let your ‘yes’ mean ‘yes,’ and your ‘no’ mean ‘no.’ Anything more than this is from the evil one.”
If I’m getting this right, heart purity is to be free of all ambivalence, all double-mindedness. It means ignoring any motive at variance with your main purpose for being where you are, saying, or doing what you’re doing at any given moment.
To that end, simply ask yourself, why am I here right now? or why am I doing this? Once that reason is clear to you, purity of heart means fusing together your main reason and your main motive for what you’re saying or doing and then ignoring alternate motives the instant they rap on the door of your mind. In effect, your whole being says ‘yes’ and means ‘yes’, or says ‘no’ and means ‘no.’
