The fundamental doctrine of Christian faith is obviously known as the justification of sinners. The doctrine of justification by faith was the core idea in religious reformation. Luther articulated that the doctrine of justification by faith is the core idea of the whole scripture. According to Luther individuals receive the gift of salvation through faith alone.
Therefore, Luther's understanding on justification is faith as the forerunner of the redemption of the sinners. John Calvin understood and took differently the focal point of justification from Luther though he had a substantial agreement with Luther. Calvin's focal point was the mercy of God (Misericordia Dei in Christo) as the efficient cause in justification but for Luther faith as the main article in which the church falls or stands.
According to Calvin, justification consists of 4 key points. 1. Mercy of God (efficient cause), 2. Jesus Christ (element cause) 3. Faith (instrumental cause). 4. Sanctification (final cause). These 4 key points are distinguished each other but not separated. Though these four key points are strongly co-related each other, the focal point is the mercy of God in which all the spiritual blessings begets. According to Calvin we are saved by the grace of God through faith (Eph. 2:8).