There is a debate within Reformed theology concerning whether election precedes covenant, or the other way around. Federal Visionists tend to argue that covenant precedes election; in other words, we become "elect" when we are baptized into the covenant body and can therefore lose our election through apostasy. On the other end, you have those like myself who believe it is essential to understand that election precedes covenant. In other words, emphasis is placed on decretal, rather than corporate, election, and we are to believe that the only reason ANYONE enters into the covenant by faith is because of an antecedent, unconditional decretal election.
A helpful verse in demonstrating this is Romans 11:5. Paul speaks of a remnant which exists according to the election of grace. The Greek text uses the preposition uses "kata" + accusative which means that the remnant exists because of, or on account of, an "election of grace." It is the Greek word for "election" that takes the accusative and the word for "grace" that takes the genitive.
In other words, the reason that this remnant is in the covenant by faith is because of an antecedent election. Election is the cause of this covenant membership, not its result. Were Paul a Federal Visionists, he could have (and likely would have) said something about an election "kata" + accusative faith. But the very definition of the remnant in this context is those who are or will be regenerate by faith on account of an antecedent election, as he makes clear in this text. We can paraphrase the sense of this text by saying that there exists a believing remnant who believes because of an antecedent unconditional election that is rooted in an antecedent, unconditional grace.
That God's calling of the unconditionally elect to faith is rooted in an antecedent, unconditional, decretal election that produces faith rather than being its result is also clear from 2 Tim. 1:9. There, we are told that God saved us and called us to faith not because of our works but because of a decretal election in eternity past. There was never a time when God was not gracious toward his elect. He sent Christ to die for the Church because he loved us in eternity past. He did not send Jesus to die in order that he might love a people who elect themselves unto eternity future.