God can use frail servants

"Human frailty (e.g., youth or difficulty in speech) is no excuse before God’s expressed will to grant a person the words to say and the opportunities to deliver them."

J. Andrew Dearman 

Jeremiah, Lamentations (NIVAC)

The IMMEDIATE PURPOSES of God's revelation are subservient to its OVERARCHING PURPOSE

It seems clear that those who were used by God in the writing of the Old Testament Scriptures did not do so with the primary thought in mind that their words, centuries down the road, would be a source of encouragement for Jews and Gentiles, united in the one body of Christ. But God’s intent in the writing of Scripture is here expressed in plain words: The immediate purposes of the prophetic ministry, while used as means to bring the Scriptures into existence, are subservient to God’s overarching purpose, which is providing the body with its chief means of encouragement and guidance. What was written “before” or in “earlier times” was written not just for those who lived then—a greater purpose was in sight, as it was written “for our instruction."

James R. White (Scripture Alone)

The Heart and Mouth Connection

"... when the heart is filled with good and noble intentions the good man’s speech will prove this to be a fact. The rule according to which whatever a man has set his heart on, so that the very core and center of his being is full of it, will sooner or later be disclosed in his speech..."

William Hendriksen
Exposition of the Gospel According to Matthew