Moreover - literally, And. Notice the connection between Jeremiah‘s call and first prophecy.
Matthew Henry
Concise Bible Commentary
Those who begin well, but do not persevere, will justly be upbraided with their hopeful and promising beginnings. Those who desert religion, commonly oppose it more than those who never knew it. For this they could have no excuse. God's spiritual Israel must own their obligations to him for safe conduct through the wilderness of this world, so dangerous to the soul. Alas, that many, who once appeared devoted to the Lord, so live that their professions aggravate their crimes! Let us be careful that we do not lose in zeal and fervency, as we gain knowledge.