This is a systematic belief but the Reformed were driven to it by Scripture. Grace is for sinners, not for the righteous. Adam was righteous before the fall but God was gracious to him after the fall. We all fell in Adam and he is gracious to his elect to justify, to sanctify, and to save them all for Christ’s sake alone. All who, by grace alone, through faith alone, are united to Jesus, the Last Adam, are credited with his perfect law-keeping. Christ earned his place and ours in heaven and that is good news indeed. Our salvation is not arbitrary. It is grounded in God’s justice and received by his grace and the promise of covenant of works testifies to that justice and to the certainty of our salvation in the covenant of grace.
It is often objected, in my experience, that the Reformed confession of the covenant of works, e.g., Westminster Confession 7.2, “II. The first covenant made with man was a covenant of works, wherein life was promised to Adam; and in him to his posterity, upon condition of perfect and personal obedience“ is the result of a “system” and not a doctrine (a teaching) derived from Scripture.
This is not true. The Reformed may be wrong in their interpretation of Scripture but interpret Scripture they did in order to reach the conclusion that there are in history two covenants, a covenant of works before the fall and a covenant of grace after the fall. Further, most of them concluded that behind those two historical covenants was the eternal, pre-temporal covenant of redemption between the Father and the Son (and implicitly the Holy Spirit) regarding the Son’s voluntary agreement to be the guarantee of the elect (the surety) and their substitute and Mediator and the Father’s agreement to give him a people and graciously to redeem them on the basis of his obedience for them.
For a time, the covenant of works fell out of favor even in Reformed circles but not, in my view, for particularly good reasons. In recent decades it has, happily, made something of a comeback. Nevertheless, there are those both within and without the Reformed world who are quite unfamiliar with the history of Reformed covenant theology, who have no idea how or why the Reformed reached their conclusion that God made with Adam a covenant of works, which was finally fulfilled by Christ the Last Adam (1 Cor 15:45).
What follows is a very brief, barebones sketch of some of the reasons that drove the Reformed to conclude that God had promised to Adam entrance into eternal life upon condition of his obedience.
First, we should not think that the Reformed invented the idea of a probation before the fall. This view was well known in the Patristic and Medieval periods of church history. See e.g., the Patristic and medieval comments on Hosea 6:7. The Reformed adopted and adapted the doctrine of Adam’s pre-fall probation to articulate the Protestant distinction between Law (as the “do this and live” principle) and the Gospel (as the announcement that Christ has done for all his people). The covenant of works stood for Law and the covenant of grace stood for gospel. One of our earlier covenant theologians, Zacharias Ursinus, made this correlation explicitly.
Second, as to the objection that the Reformed are captive to a system, while others are ostensibly reading the Bible without a system, that is simply false. Theological systems are like belly buttons: everyone has one, even if not everyone seems to be aware of it.