The title of Chandler's book originated in a blog post by Josh Patterson, one of the pastors at the Village Church. It's worth reading it's entirety:
Is the gospel assumed in your relationships? Or, is the gospel
explicit? I have been thinking about this distinction for a few days
now. Those who live life under the banner of an assumed gospel simply
navigate the waters of life with an underlying foundation that is
personal and meaningful. An assumed gospel often means that a person
deeply values the gospel and tries to live life according to the gospel.
On the contrary, those who are explicit about the gospel in their
relationships have a different effect. By living out the gospel and
speaking about the gospel and working through the gospel (verbally),
they are helping to connect the dots for those around them. Their kids
hear how the gospel relates to the family finances or time or
relationships or arguments. Their neighbors hear about the hope within.
Co-workers are privy to the reality that this person is not simply a
moral guy/girl, but one who is forgiven and transformed by the death and
resurrection of Christ.
I want to encourage you to begin, and with some of you, continue to
make the gospel explicit in your relationships. Don't waste life by
living an assumed gospel; rather, flesh it out and connect the dots for
yourself and those around you. Talk with your spouse about how Christ's
Person and work relates to everything. Pass this on to your kids.
Mention Christ. Talk about Christ. Point to Christ. Relate to Christ.
Oftentimes where the gospel is assumed, it is quickly lost.