Definition and Origin:
The phrase "grace upon grace" is derived from the Gospel of John, specifically John 1:16, which states, "From His fullness we have all received grace upon grace." This expression is a profound theological concept that encapsulates the abundance and continuity of God's grace bestowed upon believers through Jesus Christ.

Biblical Context:
In the prologue of the Gospel of John, the Apostle introduces Jesus as the Word made flesh, emphasizing His divine nature and mission. John 1:14-17 highlights the incarnation of Christ and the revelation of God's glory, grace, and truth through Him. The phrase "grace upon grace" signifies the superabundant and overflowing nature of God's grace that believers receive through Christ.

Theological Significance:
1. Abundance of Grace: The expression "grace upon grace" suggests an inexhaustible supply of divine favor. It indicates that God's grace is not a one-time gift but a continuous flow, replenishing and sustaining believers throughout their spiritual journey.

2. Fulfillment of the Law: John 1:17 contrasts the giving of the Law through Moses with the grace and truth that came through Jesus Christ. While the Law was given to guide and reveal sin, grace through Christ provides redemption and empowerment to live according to God's will.

3. Progressive Revelation: The concept of "grace upon grace" also implies a progressive revelation of God's character and purposes. As believers grow in their faith, they experience deeper levels of grace, understanding, and intimacy with God.

4. Empowerment for Christian Living: This abundant grace equips believers to overcome sin, endure trials, and fulfill their God-given purposes. It is through grace that Christians receive spiritual gifts, strength for service, and the ability to bear fruit in their lives.

Practical Implications:
1. Assurance of Salvation: The continuous nature of grace assures believers of their secure position in Christ. It underscores the sufficiency of Christ's atoning work and the believer's ongoing sanctification.

2. Encouragement in Trials: Understanding "grace upon grace" provides comfort and encouragement during difficult times. Believers can trust that God's grace is sufficient for every challenge they face, as echoed in 2 Corinthians 12:9, "My grace is sufficient for you, for My power is perfected in weakness."

3. Motivation for Holiness: The realization of God's abundant grace motivates believers to pursue holiness and live lives that reflect His character. It is not a license to sin but an empowerment to live righteously.

4. Call to Extend Grace: As recipients of divine grace, Christians are called to extend grace to others. This includes forgiveness, compassion, and acts of kindness, reflecting the grace they have received from God.

Conclusion:
The phrase "grace upon grace" encapsulates the richness and depth of God's favor towards humanity through Jesus Christ. It is a central theme in Christian theology, emphasizing the continuous and abundant nature of God's grace that empowers believers to live victorious and fruitful lives.