Romans 3: 26, “for the demonstration, I say, of His righteousness at the present time, that he might be just and the justifier of the one who has faith in Jesus.” (NASB)
The previous verse speaks to the sinners under the Law, now this verse speaks to us. Since God is Just and the Justifier we that believe are imputed with Christ’s righteousness through our faith.
Mercy and truth are so met together, righteousness and peace have so kissed each other, that it is now become not only an act of grace and mercy, but an act of righteousness, in God, to pardon the sins of penitent believers, having accepted the satisfaction that Christ by dying made to His justice for them. (Henry)
How can God be Just and maintain His Justness and be the Justifier of sinner? He judged Jesus on the Cross, 2 Corinthians 5: 21, “He (God) made Him (Jesus) Who knew no sin to be sin on our behalf, that we might become the Righteousness of God in Him (Jesus).” (NASB) In effect God did this by making Jesus serve as the Sin-Offering? Isaiah 53 is the Song of the Suffering Servant, after piecing through this passage we find that Jesus took what was us for on Himself. What was meant for you and I, God the Son said no I’ll take it because if they die, they will forever be separated from the Love of The Father. So Jesus being both God and Man, so much God til He was not man and so much man til he was not God, took the full punishment of the Law on Himself in the form Christ Jesus on Calvary‘s Cross.
The Cross was the place where this beautiful act of Love was done, which becomes the means or vehicle. The Driver or Source is Christ. And the fuel is Love. The demands of the Broken Law were satisfied at the Cross. Salvation is only in Christ and Him Crucified because is the only way God can say we are Justified and be Just in pardoning the sins of believers. The Sin Debt was paid for at Calvary by Jesus, so this is why He is the key, Acts 4: 12.