Heart preaching begins, not in trying to figure out how to get the truth to the hearts of the lost, but making sure that it reaches and comes from ours. It is a glorious truth that the Gospel is not dependent upon the power or life or motives of the preacher.
Philippians. 1:15-18 – Some indeed preach Christ from envy and rivalry, but others from good will.  The latter do it out of love, knowing that I am put here for the defense of the gospel.  The former proclaim Christ out of rivalry, not sincerely but thinking to afflict me in my imprisonment.  What then? Only that in every way, whether in pretense or in truth, Christ is proclaimed, and in that I rejoice.
But we dare not make this the norm or we would be here encouraging men to be false in their motives for the sake of winning the lost to Himself. The Bible is clear that those handling the Word of God are to work at being approved of God with no need to be ashamed. We are to watch our lives and doctrine closely with the promise that if we do, we and our hearers will be saved. Our hearts are crucial in the matter of preaching the Gospel.
Heart preaching is passion for the God of whom we preach, the Truth of the Word that He has spoken, the salvation of the lost, the sanctification and edification of the church. It is difficult to comprehend how any man can stay in the pastorate for the long haul without this passion, and perhaps this is why so many do not make it.
Heart preaching is Jeremiah in Jeremiah 20:9
If I say, “I will not mention him,
or speak any more in his name,”
there is in my heart as it were a burning fire
shut up in my bones,
and I am weary with holding it in,
and I cannot.
This, it needs to be noted, is said by Jeremiah when everything has gone south for him and he is charging God with misleading him. “If I had known things were going to turn out like this, I would never have signed on”, seems to be his attitude. But he cannot stop. He cannot stop because God has called him to preach and he cannot keep in what God has told him to let loose. Heart preaching is preaching that identifies with this in at least some measure. It is part of our response to a call to deliver the Word of God.
There are many today, as there has always been, who believe they have a message from God that everyone should pay attention to. They have a burning in their hearts, for sure, but it is not based on the Scriptures. Mormons speak about knowing they have the truth because of a “burning in the bosom” that confirms it for them. Many of us encounter Muslims who have a great passion for a false book and a false prophet. Eliphaz himself, the counsellor of Job even had divine approval, as far as he was concerned.
Job 4:12-16 (ESV)
“Now a word was brought to me stealthily;
my ear received the whisper of it.
 Amid thoughts from visions of the night,
when deep sleep falls on men,
 dread came upon me, and trembling,
which made all my bones shake.
 A spirit glided past my face;
the hair of my flesh stood up.
 It stood still,
but I could not discern its appearance.
A form was before my eyes;
there was silence, then I heard a voice:
Too much heat and not enough light is a far too common malady. It has produced a Christianity that has little use for the Word of God and little use for sound doctrine based upon that word. But it should not deter us from praying a fire into our hearts for the Scriptures from which we preach, for the God of whom we preach and for the people to whom we preach. Heat without light is nothing more than warm darkness, and we should take all legitimate measures to ensure that we do not have the disease. But we who are more prone to light without heat need to take care that we do not give people something to see while leaving them shivering. Jeremiah had the right thing and we should desire the experience for ourselves before parsing our way out of the necessity.
“It is dreadful work to listen to a sermon, and feel all the while as if you were sitting out in a snowstorm, or dwelling in a house of ice, clear but cold, orderly but killing. You have said to yourself, “That was a well-divided and well planned sermon, but I cannot make out what was the matter with it;” the secret being that there was the wood, but no fire to kindle it. A great sermon without the heart in it reminds one of those great furnaces in Wales which have been permitted to go out; they are a pitiful sight. We prefer a sermon in which there may be no vast talent, and no great depth of thought; but what there is has come fresh from the crucible, and, like molten metal, burns its way.” (Spurgeon)
We have no use for mere excitement in the pulpit. Far too often it is no more than forced, orchestrated emotionalism that is a detriment to the Gospel. But that should not make us shy away from a real movement of God upon our own hearts. Preaching in a manner that gives the impression that the truth has never lifted us to great heights of wonder is shameful. We must long for and pray for hearts that feel, and then demonstrate the wonder of the Gospel. It is a heart for God and all that falls out of that. We must be men who preach from the very marrow of our souls because our hearts will not let us keep quiet.