First of all what is the heart? The brain, where you think?
The bible often says things like "think in your heart" or "reasoned in their hearts". This sounds like it simply means the mind, where you think thoughts.
But then why do christians make a difference between heart and mind?
How can you even know wether you believe in the heart or not?
Or how can you know if you doubt in the heart or somewhere else?

1 Thessalonians 5:23 says: Now may the GOD of peace HIMSELF sanctify you completely, and may your
spirit,
soul, and
body be preserved completely whole and blameless at the coming of our LORD JESUS CHRIST. Mounce Reverse-Interlinear New Testament
Your
spirit (rauch, pneuma) is the eternal part of you that never dies and is made completely new and perfect at the moment it is fused to the HOLY SPIRIT when you give yourself to GOD and allow HIM to take permanent residence in you. It can also be considered the spiritual processing unit of your being. It spiritually processes all things and is in perfect interface with the HOLY SPIRIT at all times. It becomes (or should be allowed to become) your primary processing unit when you are a born-again child of GOD. 2 Corinthians 5:17, Galatians 2:20
Your present physical
body (sarka) is the part of you that returns to dust when you die. You get a new glorified body when you go to heaven. Philippians 3:21
Your
soul (nephesh, psyche) is comprised of your mind, will, and emotions. This would include your brain, your heart, and your stomach. All three are neural centers loaded with neurons. (Google "
neurons in the stomach" and "
neurons in the heart") They are linked together and make up your core inner being, which is really a neurological interfacing processing unit with which you mentally, emotionally, and physically process everything you experience. It is imperfect and changes with input. Input GOD'S WORD and it stabilizes. Input things that contradict GOD'S WORD and it destabilizes. Genesis 2:9, 1 Corinthians 15:45
Before you became a born-again child of GOD your
soul was your primary processor, but now that your
spirit is fused to the HOLY SPIRIT, it is better if your
soul allows your
spirit and the HOLY SPIRIT and the WORD of GOD to lead. This is a learning process.
The Bible talks about your
soul as the thoughts of your heart, the renewing of your mind, the strengthening of your inner man, etc. It is the deepest inner core part of you, that is affected by what you think, see, hear, feel, believe, say, experience, etc. The Bible tells you to guard your heart because out of it come the issues of life. Which means to guard your neural centers from anything that would negatively affect the integrity and health of your core inner being.
As far as faith and doubt, etc, you do not need to fear if you have given yourself to GOD, and continue to give yourself to HIM. The HOLY SPIRIT will never leave you, so turn to HIM. Keep learning to make your
soul follow the HOLY SPIRIT and the WORD of GOD. Lean on the HOLY SPIRIT. Talk to HIM and listen to HIM and use HIS ability rather than your own. When you pray in tongues all kinds of things you know nothing about are being perfectly dealt with by the power of the HOLY SPIRIT.
Read GOD'S WORD abundantly. It is the way to re-program the
soul so that it becomes and remains stabilized. If you are having trouble believing or understanding something GOD said in HIS WORD, give yourself to GOD and ask HIM to give you HIS ability, thoughts, faith, understanding, feelings, etc. This is called being filled with the HOLY SPIRIT daily. You cannot do anything apart from HIM so don't bother trying. Mark 9:24, Ephesians 3:16, Proverbs 3:5, 1 Peter 5:6-7, Ephesians 5:18, John 15:5
HE promised that the work HE began in you HE will completely finish. JESUS and the FATHER have you firmly in their hand, so turn to them for EVERYTHING all the time. Philippians 1:6, Hebrews 12:2, Romans 14:4, Jude 1:24, John 6:39, Hebrews 4:16
ps. As a born-again child of GOD, HE asks you to do everything in/thru/by/with HIM. You can choose moment by moment to acknowledge HIM and yield to HIM. That is called walking in the spirit (pneuma). When you choose to lean on yourself and your own ability and understanding, that is called walking in the flesh (sarka). This is a learning process.