Allow me to share a few quotes. As you read, I want you to ask yourself where, in the history of the church these quotes are found?
Gothard offered to save families not by offering to parents and children free salvation by grace alone, through faith alone but by law.
The Marrow of Modern Divinity was regarded by the orthodox Reformed, in the 17th century, as a good summary of the orthodox view of law and gospel, justification, sanctification, and the third use of the law.
The Marrow was not well received by the 18th century General Assembly of the Scottish Church. When Thomas Boston recovered and promoted it, controversy ensued.
According to Hebrews 12, Christians, i.e., those who have received, by grace alone, through faith alone, the benefits of the covenant of grace, are motivated to sanctification not by the covenant of works ('do this and live') but by the covenant of grace ('do this because Christ lived for you').
One of the best ways to understand the incarnation and the two natures of Christ is to consider the Definition of Chalcedon and the Athanasian Creed.
What is in question is the wisdom and truth of using the gospel as a way of leveraging a discussion in Christian ethics.