The outward covenant
The next Tabletalk article (October issue) is on the history of the covenant (pg 8-11, and is also quite good. It seems to turn into a brief against Federal Vision people, though. Here is the end:
"According to them [Fed Vis guys], every baptized person is elect and united to Christ through baptism, but this election and union can be forfeited through faithlessness."
I haven't read many FV guys - mainly just one. But I would agree with the above quote if the words elect and election were removed. We are baptized into union with Christ, but can be broken off if we don't bear fruit (John 15). This doesn't mean we were saved or elect, just part of the visible church for a while. This is probably is a bit more "high view of sacraments" than most Reformed people are comfy with - majority would say baptism is a sign of union with Christ, not that baptism actually does that. The sticky point is that we don't know who is elect, and God has called us to act this way, so by faith we ought to believe what He says in His word about the baptized being buried with Him in baptism, unless their fruit later in life shows otherwise.
Ironically, the same article, a column earlier, summarizes this quite well as it lays out Caspar Olevianus' position (author of Heidelberg Catechism):
"Since only God knows who is elect... the covenant of grace... can be said to be with all the baptized. Therefore we baptize on the basis of the divine command and promise, and we regard covenant children (before profession of faith) and all who make a credible profession of faith as Christians until they prove otherwise. Those who are in the covenant only in this broader sense or externally, do receive some of the benefits of the covenant (Heb 6:4-6), but they do not receive what Olevian called the 'substnace of the covenant:' justification and sanctification. Only those who are elect actually appropriate, by grace alone, through faith alone, the 'double benefit' of the covenant of grace."