The apostle John writes to Gaius as a father to his son, praying that he may prosper.
“2 Beloved, I pray that you may prosper in all things and be healthy, even as your soul prospers. 3 For I rejoiced greatly when brothers came and testified about your truth, even as you walk in truth. 4 I have no greater joy than this: to hear about my children walking in truth. 3 John 1:1-4
John prays that Gaius will prosper as his soul prospers.
He had been informed that Gaius’s soul was in a prosperous condition.
That indicates John is not writing about money because our soul is intangible. Our soul lives eternally, either in heaven or hell. It is the seat of our mind, emotions and will. See 100 new testament verses
Heart is closely related to how our soul prospers.
Biblical usage:“The heart is that organ in the animal body which is the center of the circulation of the blood, and hence was regarded as the seat of physical life. It denotes the center of all physical and spiritual life, the vigor and sense of physical life, the center and seat of spiritual life, the soul or mind, as it is the fountain and seat of the thoughts, passions, desires, appetites, affections, purposes, endeavors, of the understanding, the faculty and seat of the intelligence, of the will and character, of the soul so far as it is affected and stirred in a bad way or good, or of the soul as the seat of the sensibilities, affections, emotions, desires, appetites, passions, of the middle or central or inmost part of anything, even though inanimate.” See 160 NT verses
Our soul has been called out of darkness into God’s marvelous light.
“But you are a chosen race, a royal priesthood, a holy nation, a people for God’s own possession, that you may proclaim the excellence of him who called you out of darkness into his marvelous light. In the past, you were not a people, but now are God’s people, who had not obtained mercy, but now have obtained mercy.
“Beloved, I beg you as foreigners and pilgrims to abstain from fleshly lusts which war against the soul, having good behavior among the nations, so in that of which they speak against you as evildoers, they may see your good works and glorify God in the day of visitation. (The day of visitation is the day receiving Messiah Jesus.) 1 Peter 2:9-12
Scriptures are from the World Messianic Bible.