A prosperous soul is one in whom the truth dwells, and dwells richly. You must have remarked, in reading the first eight verses, how much the apostle Paul makes of truth. He describes Gaius as having the truth dwelling in him, as walking in the truth, as beloved for the truth’s sake, and as being a fellow helper of the truth. All these expressions are found in those verses. It seems then to enter into the very essence of a prosperous soul, that the truth dwelt in him, and that it dwelt richly in him. Truly, my brethren, gospel truth is that to the soul which wholesome food is to the body, and wholesome words and sound doctrine have on effect on the soul similar to that which wholesome food has on the body; they render it strong, vigorous, and active. Thus the great principles of evangelical truth being imbibed by Gaius, afforded a constant spring of activity. He was a lively, active, generous man. It is of great importance what principles we acquire. Principles will be active—will be influential. Indeed this is the very reason why Divine truths are called principles. We read of the first principles of the doctrines of Christ, and principles you know signify the first moving causes which lie at the foundation and source of action. Merely speculative notions or speculative ideas, that have no influence on a man’s heart, are not principles; they may be called more properly opinions; but if the truths of God are imbibed as a thirsty man would drink in water from a fountain, they become in him a well of living water springing up in the disposition to do good, and terminating in everlasting glory. Principles, whether good or evil, will be influential if they are thoroughly imbibed. Hence we read of false doctrines having a fatal influence. The Scripture speaks of God giving men up to strong delusion, or to the energy or efficacy of deception or error.
All principles, if they deserve the name of principles, lie at the bottom and source of affections and actions. If they be genuine, evangelical, and true, they are the spring of a holy life, and lie at the bottom of evangelical obedience; but if they be false principles, they lie at the bottom of a course of alienation and apostacy from God. Indeed, as right principles stimulate to right actions, so where a person imbibes wrong principles, or is indifferent to right, it enervates right actions: even good men, who have swerved in a greater or less degree from the truth, have sunk into a spirit of indifference with regard to evangelical principles—it has had the effect of stagnating their souls in Divine actions.
Excerpt from: “Soul Prosperity (3 John 2),” in Sermons and Sketches.
Fuller, A. G. (1988). The Complete Works of Andrew Fuller: Memoirs, Sermons, Etc. (J. Belcher, Ed.) (Vol. 1, p. 405). Harrisonburg, VA: Sprinkle Publications.