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Christ as Mediator entered the representative relation in which Adam was in the integrity state, in order to deserve for the sinner the eternal life. This constitutes the active obedience of Christ who consists of everything what He in his representative aspect did to obey the law, like condition to obtain eternal life. The active obedience of Christ was necessary so that his passive obedience was acceptable to God, that is to say, to turn it into object of the God's approval. Only bearing in mind this, God estimates Christ's sufferings in form different from the one that estimates the sufferings of the lost ones..... And finally, if Christ had suffered nothing more that the punishment imposed on the man, which take part of the fruits of his work would have stayed in the exact place where Adam was before the fall.

God kept on demanding obedience of the man (after the fall), but in addition it needed that from him that he was paying the punishment of the past disobedience. To satisfy this double request was the only way of the life after the sin entered the world. If Christ was nothing more obeyed the law and had not paid the punishment, it could not have gained a title to eternal life for the sinners; and if he had paid only the punishment, without paying the original demands of the law, it had left the man in the position in which Adam was before the fall facing still the task of obtaining the eternal life in the way of the obedience. Nevertheless, Christ by means of active obedience took his people beyond that point and gave them right to life eternal.

Taken of the Systematical Theology of Berkhof, Págs. 451, 452 and 453