1 Corinthians 3:7, ". . . So then neither is he that planteth any thing, neither he that watereth; but God that giveth the increase. . . ." Compare and note John 17:17, Romans 10:17 and Ephesians 1:12-14 ". . . Christ . . . In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise, . . ."
You didn’t say what the role of the preacher was in someone’s conversion but I think you will understand what I am saying here. God used something physical (the Preacher) as an “instrument” to bring about something Spiritual. When a person comes to Christ through the preached Word, the preacher is the “instrumental cause” of the grace of conversion. The grace comes from God but it was through the “Instrument” of the preacher. This is exactly what Baptism is. In general, this is the definition of a sacrament. The definition of a sacrament: “A sacrament is an outward sign, instituted by Christ, that gives grace.” That’s the definition from the Baltimore Catechism. In scripture most of the time it translates as a “mystery” or “Secret”. I would interpret it as “hidden” but strongs interprets it as “silent” (strongs G3466). That doesn’t mean that when the word “mystery” or “secret” is used in scripture that it refers to a sacrament. They used the word to define a new reality that Christ instituted. One example that stands out to me is Eph 5:32 which refers to the Sacrament of Matrimony:
“This is a great
sacrament: but I speak in Christ and in the church.” (DRV)
“This is a great
mystery, and I mean in reference to Christ and the church.” (KJV)
St. Jerome translated it as “Sacramentum” in the fourth century in the Vulgate.
Physical things are used as “Instruments” to bring about Spiritual things (grace)
If you think about it, when you pray you are using something physical (yourself) to bring about a spiritual result (grace). You are the “instrumental cause” of the the grace that comes from your prayers. This is basically the definition of a sacrament. It is where the physical and the spiritual come together. The Catholic Church narrows the definition of a sacrament to include only those physical things used to produce spiritual results that Christ instituted. The term “mystery” has a wider use of the physical and spiritual coming together.