The meaning of the words of Allah, “and know that Allaah comes in between a person and his heart” [al-Anfaal 8:24]

The verse means what it appears to mean, which is that
Allah, may He be glorified and exalted, controls the affairs of His
slaves. So He may guide one person and open his heart to faith and guide
him to Islam, or He may place some reluctance in his heart and make him
uneasy about the religion of Allah, which prevents him from coming to
Islam, so He comes in between a person and his heart, as it says in the
verse [interpretation of the meaning]: “And whomsoever Allaah wills to
guide, He opens his breast to Islam; and whomsoever He wills to send
astray, He makes his breast closed and constricted, as if he is climbing
up to the sky” [al-An ‘aam 6:125]. 

So Allah is the One Who controls the affairs of His slaves
however He wills. So He opens the heart of one person to faith and guidance,
whilst another person is not helped and guided to that. End quote. 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him)