Put simply, in Reformed theology, salvation is the application or administration of Christ's redemptive work to the elect, while justification is the part of that application in which the sinner is declared righteous.

Salvation

The Reformation Study Bible does a nice job of showing how broad salvation is:

Salvation delivers the believer from the wrath of God, the dominion of sin, and the power of death. God liberates sinners from the natural condition of being mastered by the world, the flesh, and the devil. He frees believers from the fears that a sinful life generates, and from the vicious habits that enslaved them. Salvation brings not only the promise of spiritual wholeness and peace, but also of physical healing. Although Christians have already received salvation, they will experience the benefits of salvation in their fullness only when Christ returns at the end of the age.

There's a lot here to unpack, and one of the distinctives of Reformed theology is the ordo salutis, or the "order of salvation," which Berkhof defines:

The ordo salutis describes the process by which the work of salvation, wrought in Christ, is subjectively realized in the hearts and lives of sinners. It aims at describing in their logical order, and also in their interrelations, the various movements of the Holy Spirit in the application of the work of redemption.

There is some disagreement among Reformed theologians as to the exact elements and ordering of the ordo salutis, but a common layout is:

• Election
• The gospel call (or effectual call)
• Regeneration
• Conversion
• Justification
• Adoption
• Sanctification
• Perseverance
• Death
• Glorification

As you see, justification is one of the aspects within the bigger picture of salvation.

Justification

So what, then, is justification? Broadly speaking, it means "to be declared just or righteous." Sometimes, such as in James 2:21, it considers the actual righteous condition of someone ("the just is declared just"). More frequently in the NT, it refers to the imputation of one person's righteousness to another.

Thus Berkhof defines it:

Justification is a judicial act of God, in which He declares, on the basis of the righteousness of Jesus Christ, that all the claims of the law are satisfied with respect to the sinner.

It's important to note that it is a judicial act:

It is unique in the application of the work of redemption in that it is a judicial act of God, a declaration respecting the sinner, and not an act or process of renewal, such as regeneration, conversion, and sanctification. While it has respect to the sinner, it does not change his inner life. It does not affect his condition, but his state, and in that respect differs from all the other principal parts of the order of salvation. It involves the forgiveness of sins, and restoration to divine favor.

As a judicial act, justification simply removes the guilt of sin, and does not change the inner life of the justified. It is a single act (not a process) that occurs only once, so a person cannot be "partially justified."

Interactions with other elements of the ordo salutis

Though justification is viewed narrowly by Reformed theologians as compared to some, it is tightly connected with other elements in the ordo salutis. For example, justification and adoption are inseparable – the removal of guilt in justification corresponds closely with the restoration of a sinner to a right relationship with God.

Similarly, the process or work of sanctification closely follows the act of justification. Defined in the Westminster Shorter Catechism:

Sanctification is the work of God's free grace, whereby we are renewed in the whole man after the image of God, and are enabled more and more to die unto sin, and live unto righteousness.

In other words, though justification does not in itself involve a change of heart or behavior, its immediate successor, sanctification, is a process – not an "act" like justification – through which a sinner is freed from sin, renewed in his nature, and enabled to perform good works.

Summary

Within the ordo salutis, as viewed by the Reformed, the act of justification takes a central role – without it, salvation would not be possible. God imputes Christ's righteousness to the believer's account and declares him guiltless with respect to the demands of the law.

This declaration of righteousness necessarily follows election and regeneration, two other steps in the ordo salutis, and it is necessarily followed by adoption and sanctification, through which the believer comes into a right relationship with God and grows in his Christian walk.

References

• Berkhof, Systematic Theology
  • 4.1, Ordo Salutis
  • 4.9, Justification
  • 4.10, Sanctification
• Grudem, Systematic Theology, Chapter 32, 670. The order of salvation included here comes from Grudem.
• Reformation Study Bible, 1680
• Westminster Shorter Catechism, Answer 35