WCF Chapter 8, Of Christ the Mediator, speaks directly to what Boaz pictures. The confession affirms that the Lord Jesus, by his perfect obedience and sacrifice of himself, fully satisfied the justice of his Father, purchased not only reconciliation but an everlasting inheritance in the kingdom of heaven for all those given to him by the Father. Boaz willingly assumes the cost of redemption so that Ruth receives the blessing. Christ does the same, and infinitely more, taking on the full weight of what we owed so that we might receive what we could never earn.
WCF Chapter 3, Of God's Eternal Decree, frames the sermon's observation about who ends up in Christ's lineage. Ruth the Moabite and Rahab the harlot are not accidents in the genealogy of Jesus. They are the decree made visible. God's electing love does not run along the lines of human expectation, bloodline, or reputation. He redeems sinners according to his own sovereign grace, and the lineage of Christ is the most public display of that truth in the entire Old Testament.
WCF Chapter 16, Of Good Works, provides the framework for the sermon's distinction between legalism and true Christian obedience. The confession affirms that good works are the fruits and evidences of a true and lively faith, done in obedience to God's commands, out of a sincere love for him, not to establish our own righteousness but because we have been freely given one. We are not redeemed by our obedience. We are redeemed unto it.