The paradox of true saving faith being both a simple matter and an impossibility lies with it being a matter of desire. A thing can be impossible because it is a thing which cannot be done even if desired: I cannot fly like a bird or swim like a fish, no matter how greatly I would desire otherwise. I cannot be ten feet tall or hold my breath for a month. I cannot meet King Solomon nor know the actual thoughts of another human being. Those things are impossible irrespective of my desire.
Other things are impossible because I do not desire them. For most people, a vicious crime would impossible, because they could not desire the injury of another human being. When I was a single man, I could know a woman and yet not “love” her or desire her in any romantic manner. Such a desire for her would be “impossible” – even though human beings romantically desire one-another all the time. Conversely, it would be equally impossible for me to not love my wife or children. Or, many children know all about various vegetables and have no desire for the vegetable. In each of these instance, the actual behavior is perfectly possible – and yet it is impossible due to a lack of corresponding desire.
The real motor which controls the human heart is desire: We do what we want/desire to do.
The effect of God upon the heart is to convey desire, love, affection: The propositional content of the Gospel is conveyed. In many instances, nothing happens because there is no lack of corresponding desire. However, when the Spirit operates with the words of the Gospel, the effect is to create desire for God in Jesus Christ.
As Guthrie writes:
Now, I say, this acting of the heart on Christ Jesus is not so difficult a thing as is conceived. Shall that be judged a mysterious difficult thing which does consist much in desire? If men have but an appetite, they have it; for they are ‘blessed that hunger after righteousness.’ (Matt. 5: 6.) ‘If you will,’ you are welcome. (Rev. 22: 17.) Is it a matter of such intricacy and insuperable difficulty, earnestly to look to that exalted Saviour: ‘Look unto me and be ye saved, all the ends of the earth.’ (Isa. 45: 22.) And to receive a thing that is offered, held forth, and declared to be mine, if I will but accept and take it, and in a manner ‘open my mouth,’ and give way to it? ‘Open thy mouth wide and I will fill it.’ (Psa. 81: 10.)
Incidentally, this is an essential element in discipleship, biblical counseling, and sanctification: Change occurs as a result of desire change. When one desires something better than sin or better than old ways of thinking and acting, the new behavior or action is effected.