Dishonesty of heart is that on account of which men receive not the word of God, so as to bring forth fruit. This is fully implied in the parable of the sower, recorded in the eighth chapter of Luke. The reason why those hearers represented by the good ground received the word, and brought forth fruit, rather than the others, was that they had “good and honest hearts;” plainly intimating that the reason why the others did not so receive it was that their hearts were not upright before God. Indeed, such is the nature of Divine truth, that every heart which is honest towards God must receive it. An honest heart must needs approve of God’s holy law, which requires us to love him with all our powers; and this because it is no more than giving him the glory due to his name. An honest heart will approve of being justified wholly for Christ’s sake, and not on account of any of its own works, whether legal or evangelical; for it is no more than relinquishing a claim which is justly forfeited, and accepting as a free gift that which God was under no obligation to bestow. Further, An honest heart must rejoice in the way of salvation as soon as he understands it, because it provides a way in which mercy can be exercised consistently with righteousness. A right spirit would revolt at the idea of receiving mercy itself in a way that should leave a blot upon the Divine character. It is the glory of Christ that he has not an honest man for an enemy. The upright love him.
We are not ignorant who it is that must now give men honest hearts, and what is the source of every thing in a fallen creature that is truly good; but this does not affect the argument. However far sinners are from it, and whatever Divine agency it may require to produce it, no man who is not disposed to deny the accountableness of creatures to the God that made them will deny that it is their duty; for if we are not obliged to be upright towards God, we are obliged to nothing; and if obliged to nothing, we must be guiltless, and so stand in no need of salvation.
Excerpt from: Excerpt from: “Proposition III: Though the Gospel, Strictly Speaking, is not a Law, But a Message of Pure Grace; Yet it Virtually Requirers Obedience, and Such an Obedience as Includes Saving Faith,” in The Gospel Worthy of All Acceptation.
Fuller, A. G. (1988). The Complete Works of Andrew Fuller: Controversial Publications. (J. Belcher, Ed.) (Vol. 2, pp. 355–356). Harrisonburg, VA: Sprinkle Publications.