Charles H. Spurgeon on Psalms 86:11

Unite my heart, etc. Sincerity drives but one design, and that is to please and enjoy God; and what can more establish and fix the soul in the hour of temptation than this? The reason why the hypocrite is unstable in all his ways, is given us by the apostle: he is "a double minded man", a man of two souls in one body; as a profane wretch once boasted, that he had one soul for God, and another for anything. But all the designs of a gracious heart are united in one; and so the entire stream of his affections runs strong. It is base by ends and self interests, that, like a great many ditches cut out of the bank of a river, draw away the stream out of its proper channel, and make its waters fail. But if the heart be united for God, then we may say of such a Christian, as was said of a young Roman, "What he does is done with all his might." A man of only one design, puts out all his strength to carry it; nothing can stand before him. Sincerity brings a man's will into subjection to the will of God; and this being done, the greatest danger and difficulty is over with such a man. This is that holy oil which makes the wheels of the soul run nimbly, even in the difficult paths of obedienee. —John Flavel.