In particular these specific verses are not in the New International Version (NIV) main text.
Note: In the New International Version, the following verses are absent from the main text, but have been translated for inclusion in the footnotes (fn).
It begins with prayer to God (as Paul lays out in Philippians 4:6-7) and overflows into communication with the other person.
In other words, the key to guarding your heart is to talk to God about the relationship before you talk to the other person about the relationship. First, hearts become unguarded when you move too fast in the relationship–becoming too vulnerable too quickly.
Next was this worrying verse endorsing genocide, from 1 Samuel 15:3: "This is what the Lord Almighty says ...