Charles H. Spurgeon on Psalms 66:18

Whence is it that a man's regarding or loving sin in his heart hinders his prayers from acceptance with God?

1. The first reason is, because in this case he cannot pray by the Spirit. All prayers that are acceptable with God are the breathings of his own Spirit with us. Ro 8:26. As without the intercession of Christ we cannot have our prayers accepted, so without the intercession of the Spirit we cannot pray...

2. The second reason is, because as long as a man regards iniquity in his heart he cannot pray in faith; that is, he cannot build a rational confidence upon any promise that God will accept him. Now, faith always respects the promise, and promise of acceptance is made only to the upright: so long, therefore, as men cherish a love of sin in their heart, they either understand not the promises, and so they pray without understanding, or they understand them, and yet misapply them to themselves, and so they pray in presumption: in neither case, they have little cause to hope for acceptance...

3. The third reason is, because while we regard iniquity in our hearts we cannot pray with fervency; which, next to sincerity, is the great qualification of prayer, to which God has annexed a promise of acceptance (Mt 11:12): "The kingdom of heaven suffereth violence, and the violent take it by force." Mt 7:7: those only that seek are like to find, and those that knock to have admittance; all which expressions denote vehemence and importunity. Now, the cause of vehemence, in our prosecution of any good, is our love of it; for proportionable to the affection we bear to anything is the earnestness of our desires and the diligence of our pursuit after it. So long, therefore, as the love of sin possesses our hearts, our love to spiritual things is dull, heavy, inactive, and our prayers for them must needs be answerable. O the wretched fallacy that the soul will here put upon itself! At the same time it will love its sin and pray against it; at the same time it will entreat for grace, with a desire not to prevail: as a father confesses of himself, that before his conversion he would pray for chastity, with a secret reserve in his wishes that God would not grant his prayer. Such are the mysterious, intricate treacheries by which the love of sin will make a soul deceive and circumvent itself. How languidly and faintly will it pray for spiritual mercies; conscience, in the meanwhile, giving the lie to every such petition! The soul, in this case, cannot pray against sin in earnest; it fights against it, but neither with hope nor intent to conquer; as lovers, usually, in a game one against another, with a desire to lose. So, then, while we regard iniquity, how is it possible for us to regard spiritual things, the only lawful object of our prayers? and, if we regard them not, how can we be urgent with God for the giving of them? And where there is no fervency on our part, no wonder if there is no answer on God's. Robert South, 1633-1716.