The true Christian character is an exhibition of state of the heart.
The proof of your heart with God is that you are doing His will.
God could testify of David as a man who will do His will because his heart was fully with God.
“…A man after mine own heart, which shall fulfil all my will.” Acts 13:22 (Isaiah 44:28)
If you are not keen about obeying God to the letter, then your heart is not with him.
Your heart is not with God by your words but by your attitude and by your action.
Saul’s heart betrayed Him by not doing God’s command. 1 Samuel 13:14.
Passion to obey God’s word makes you a man panting after His heart.