We must begin by acknowledging that conversion in either respect - either in terms of fundamental redirection or in terms of full maturity - cannot be accomplished by our own powers of persuasion.
It was Newton, furthermore, who penned the famous lyrics to "Amazing Grace" that underscore this point in the second stanza:
'Twas grace that taught my heart to fear,
And grace my fears relieved;
How precious did that grace appear,
The hour I first believed!'
Francis Xavier, George Whitefield and Billy Graham, to consider three of the most widely heard preachers in history, certainly, did not make converts of everyone in their sometimes vast audiences. The Apostle Paul himself did not convince everyone he addressed. The Lord Jesus Christ was crucified by people among whom were those quite familiar with his teaching, and even on the mount of the Great Commissioning there were some who doubted (Matthew 28:17). Apologetics and all other such Christian speech cannot in fact accomplish very much when it comes to conversion. We can pause to recognize that we cannot effect conversion even in ourselves. Spiritual adepts throughout the ages warn us that mere argument accomplishes little even within our hearts.
Among the most distinguisesd commentators on matters of the spirit was the eighteenth-century American pastor Jonathan Edwards. In his classic discussion of spiritual well-being, his Treatise on the Religious Affections, he writes:
Great use may be made of external arguments; they are not to be neglected, but highly prized and valued; for they may be greatly serviceable to awaken unbelievers, and bring them to serious consideration, and to confirm the faith of true saint: yea they may be in some respects subservient to the begetting of a saving faith in men. [Yet] ... there is no spiritual conviction ... but what arises from an apprehension of the spiritual beauty and glory of divine things."
And such a direct apprehension is a gift mediated only by the Holy Spirit of God.
When Simon Peter exclaimed that he knew that Jesus was the Messiah, the Son of God, when his fellow Jews could reach only as high as their available category of "great prophets" to describe him, Jesus pronounced: "Blessed are you, Simon son of Jonah! For flesh and blood has not revealed this to you, but my Father in heaven" (Matthew 16:17)