The focus of Lord's Day 6 is the type of and identity of the mediator who can make satisfaction between God and sinful humans. Q.16 establishes that the mediator must be human, since it was a human who sinned, and also the promised Genesis 3:15 victor; Q.17 establishes that the mediator must be God in order to provide a ransom of sufficient value and in order to sustain the infinite wrath of God and punishment for sin. Q.18 blatantly asks who this mediator is, and Q.19 asks how we know this. While the answers are straightforward -- respectively, Jesus Christ, and the Gospel -- the discussion explores the role and office of mediator, covenant and testament, and finally the nature of the Gospel message.
An interesting Objection (typically there are sections throughout the commentary of objections that might be raised, with an accompanying answer - a catechism within the catechism's commentary) is raised that "the party offended cannot be the Mediator" - specifically that Christ cannot be God if he is also the mediator. The Answer is that this is only the case if one does not recognize the distinctions (i.e., persons) within the Godhead. The Exposition of Q.18 addresses similar issues, basically using an Athanasian Creed approach in exploring the persons of the Godhead to prove that Christ Jesus, God the Son, is the acceptable and chosen Mediator. Other roles of the Son include being an ambassador to reveal to the world the Father's will, and also to present the gifts that he bestows upon his people.
Mediator is also discussed as both a role and an office. The "office" of mediator is the official position and the duties that must be accomplished to effect reconciliation, whereas "role" has to do with the specific person who fills the office.
An interesting question raised is whether Adam would have had needed a mediator had he not sinned. The answer is in how we understand the office of mediator. If it is someone through whom God communicates and bestows his benefits, then Yes, Adam would also have required a mediator. If we understand mediator more narrowly, as a redeemer, then Adam would not have needed a mediator.
Ursinus' discussion of mediator covers most of the tasks and roles that one might expect: interceding, offering, making satisfaction, but an interesting task of a mediator that most Christians would not consider is being a surety (providing a guarantee) that the offending party will not repeat the offense! Surely this is dependent on our union-with-Christ status. Not only does Christ make full satisfaction to restore our relationship with God but, hidden in Christ and one with Christ, we receive all of his benefits - as though we had never sinned and as though we no longer sin.
Ursinus also notes that mediation requires the consent of the parties involved. While not referenced directly, this speaks to the ordo salutis (order of salvation), with regeneration and faith being the first steps in a taking hold of the riches of Christ's benefits to us.
Mediation may be the process that brings two parties together, but the substance of the mediation depends on the covenant, or agreement and expectations, between the parties. Ursinus states that the mediation that we are concerned with involves one covenant in two testaments. The Bible's story of redemption starts "in the beginning" and continues, albeit with numerous twists and turns, unbroken through to its final chapter in Revelation. The promises of the first Testament are fulfilled in the second Testament. People, whether in the Old or the New, become part of the kingdom and family of God in the same way - trusting and living in faith into the promises of God's covenant. While covenant and testament can be used interchangeably, in common usage "testament" refers to a change based on a death. Christ's incarnation and death signals the shift from the Old Testament to the New Testament. One might recall Jesus' words at the Last Supper - "this cup is the new testament in my blood" (Luke 22:20). The two testaments, or testamental periods, do differ in such things as temporal blessings, rites, clarity, duration, obligation and extent (to whom it is available).
Finally, the question is raised: how we know of the Covenant. It is communicated through God's special revelation to us, commonly know as the Gospel, in the Bible. The message has become more clear since it was first given in Genesis, expanded upon by the Old Testament prophets, and finally bursting on the scene at just the right time (Gal. 4:4), to be preached to us now in its Gospel fullness.