Here we have the third instance of imputation. The first was the imputation of Adam's sin to us; and the second was the imputation of our sins to Christ. The third is now the imputation of Christ's righteousness to us. In each of the former cases, we have sought to show that the legal relation presupposes a natural relation. Adam's sin is imputed to us, because we are one with Adam; our sins are imputed to Christ, because Christ is one with humanity. So here, we must hold that Christ's righteousness is imputed to us, because we are one with Christ. Justification is not an arbitrary transfer to us of the merits of another with whom we have no real connection. This would make it merely a legal fiction; and there are no legal fictions in the divine government.
Instead of this external and mechanical method of conception, we should first set before us the fact of Christ's justification, after he had borne our sins and risen from the dead. In him, humanity, for the first time, is acquitted from punishment and restored to the divine favor. But Christ's new humanity is the germinal source of spiritual life for the race. He was justified, not simply as a private person, but as our representative and head. By becoming partakers of the new life in him, we share in all he is and all he has done; and, first of all, we share in his justification. So Luther gives us, for substance, the formula: "We in Christ = justification; Christ in us = sanctification." And in harmony with this formula is the statement quoted in the text above from Edwards, Works, 4:66.
See also H. B. Smith, Presb. Rev., July, 1881--"Union with Adam and with Christ is the ground of imputation. But the parallelism is incomplete. While the sin of Adam is imputed to us because it is ours, the righteousness of Christ is imputed to us simply because of our union with him, not at all because of our personal righteousness. In the one case, character is taken into the account; in the other, it is not. In sin, our demerits are included; in justification, our merits are excluded." For further statements of Dr. Smith, see his System of Christian Theology, 524-552.
C. H. M. on Genesis, page 78--"The question for every believer is not 'What am I?' but 'What is Christ?' Of Abel it is said: '_God testified of his gifts_' (_Heb. 11:4_, A. V.). So God testifies, not of the believer, but of his gift,--and his gift is Christ. Yet Cain was angry because he was not received _in his sins_, while Abel was accepted _in his gift_. This was right, if Abel was justified in himself; it was wrong, because Abel was justified only in Christ." See also Hodge, Outlines of Theology, 384-388, 392; Baird, Elohim Revealed, 448.