It seems that the means God has designed for evangelism is to call out the visible church by the ministry of the word.
I believe that two questions in particular from the
Shorter Catechism speak to this issue:
Q. 89. How is the Word made effectual to salvation?
A. The Spirit of God maketh the reading, but especially the preaching, of the Word, an effectual means of convincing and converting sinners, and of building them up in holiness and comfort, through faith, unto salvation.
Q. 90. How is the Word to be read and heard, that it may become effectual to salvation?
A. That the Word may become effectual to salvation, we must attend thereunto with diligence, preparation, and prayer; receive it with faith and love, lay it up in our hearts, and practice it in our lives.
I don't see "personal testimonies" playing a role in this. I'm also reminded of the account of the rich man and Lazarus in Luke 16 -
"Then he said, I pray thee therefore, father, that thou wouldest send him to my father's house:
"For I have five brethren; that he may testify unto them, lest they also come into this place of torment.
"Abraham saith unto him, They have Moses and the prophets; let them hear them.
"And he said, Nay, father Abraham: but if one went unto them from the dead, they will repent.
"And he said unto him, If they hear not Moses and the prophets, neither will they be persuaded, though one rose from the dead."
-- Luke 16:27-31 (A.V.)