And one of the scribes came up and heard them disputing with one another, and seeing that [Jesus] answered them well, asked him, “Which commandment is the most important of all?”  Jesus answered, “The most important is, ‘Hear, O Israel: The Lord our God, the Lord is one.  And you shall love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.’ – Mark 12:28-30
Heart
I confess, my friend, that I would rather begin with the mind.  As the question – let us say, the doubt – of God’s existence first entered my mind, it afflicted my heart.  And it was by way of the mind that my heart was rescued.  I want to spring to the mind, and everything else can be a footnote. Yet this saying of Jesus struck me.  To form my treatise on the words of Jesus himself as he gave the greatest commandment – it is all too fitting.  First, see:  The commandment is to love.  To love comes most naturally to the heart – even the unbelievers accept this. Second, you once expressed disdain for the idea that anyone should love God above all, even above his own children.  But I hear these words of the Lord and they are solid as stone, capable of burying a man and of elevating him.  Let us see, then, what we can build upon them.   The heart has its reasons of which reason knows nothing.   -Blaise Pascal Indeed, I am too quick to brush past the heart.   Think of your son and your daughters, for instance.  Now imagine a superintelligence, who knows reason and not the heart.  This mind presses upon you an argument which you cannot answer, which utterly compels you to abandon your children.   It would not only be permissible to do so, for any reason at all; the argument actually demonstrates that it is the best possible action, that you must abandon your children, for their greater good. The question is not, “Would you?”  The question is, “Would your heart object?” Yes.  Yes, and the heart would rather be pulled up by its roots than consent to such an act.  Likewise say the martyrs.