So, what is a sacrament after all?

Dear me, not figured it out yet? After all, I am an amateur. There is a huge difference between an amateur and a professional. Primarily that thinking is a luxury for me for which I seldom have time.
When earlier in the year I was reading Genesis, I was struck by the sacramental nature of covenant relationship. Noah gets a rainbow, Abraham gets a sighting of the land, gets circumcision, gets that ritual with God walking through the cut animals, and Jacob gets his ladder. Each time God acts, He provides a sign of that of which He has spoken, which is symbolic, that is, it is itself illustrative of the promise, and which is also a seal, a guarantee of that of which God has spoken. When the patriarchs acts in faith, they find God truly faithful to these promises.
Here's my problem: the idea of a seal. Faithlessness delays divine action, but He works out His purpose in the end. So here's the big issue: are baptism and Communion given to the church, signs symbolic of His work among us which all visible members receive (fine, happy so far) and seals that He does so act among us. Or are they seals to the individual as well? If so, what is the role of faith? "Do those without faith also receive the signified?" - the great Reformational debate - still ought be discussed after you've dismissed substantial interpretations of sacraments (Roman, Lutheran and Zwinglian) for covenantal ones (Reformed). I hope that the book I'm reading at the moment, by Leonard Vander Zee, clears that one up for me.