to him who does not work but believes on Him who justifies the ungodly, his faith is accounted for righteousness, just as David also describes the blessedness of the man to whom God imputes righteousness apart from works (Rom 4:5-6)The power of salvation is in that message – the gospel. And it is that message of justification received through faith, looking to Jesus Christ and not our experience, that communicates Christ and all his benefits. Justified through faith we are united to Christ by that same faith. But the focus of our faith isn’t our union with Christ. We are united to Christ through faith in him. Faith, enlivened in us by the Spirit's effectual call, looks to Christ. Imputed with Christ's righteousness we are justified through faith and united to him.
Walter Marshall (The Gospel Mystery of Sanctification):
“Therefore, saving faith must necessarily contain two acts, believing the truth of the gospel, and believing on Christ, as promised freely to us in the gospel, for all salvation. By the one, it receives the means in which Christ is conveyed to us; by the other, it receives Christ Himself, and His salvation in the means, as it is one act to receive the breast or cup in which milk or wine are conveyed, and another act to suck the milk in the breast and to drink the wine in the cup… Blessed are they that hunger and thirst after righteousness, for they shall be filled (Matt. 5:6). The former of these acts does immediately unite us to Christ, because it is terminated only on the means of conveyance, the gospel; yet it is a saving act, if it be rightly performed, because it inclines and disposes the soul to the latter act, whereby Christ Himself is immediately received into the heart. He that believes the gospel with hearty love and liking, as the most excellent truth, will certainly with the like heartiness believe on Christ for salvation.”The means of communicating salvation to dead sinners and nourishing God's people is the gospel which is news of a divine historical event having nothing directly to do with our subjective experience. And the gospel message is not the message of our union with Christ, which faith cannot properly have as its object. The object of faith is Christ Jesus alone and what he did to save the elect. He is the good news of God's redemptive act of grace for sinners accomplished on the cross. And he is received by the gift of faith as God effectually calls each one. Through believing the good news that Christ died for sinners, one believes not only the good news of forgiveness and righteousness imputed through faith alone but at the same time receives Christ himself. Trusting in Christ as he is presented in the gospel, by the working of the Holy Spirit, believers are united to Christ. Look away with the eyes of faith to Jesus alone, the author and perfecter of faith.
WSC Q. 86. What is faith in Jesus Christ?
A. Faith in Jesus Christ is a saving grace, whereby we receive and rest upon him alone for salvation, as he is offered to us in the gospel.