1.) Objection: “Christ needed His active obedience for Himself as a man. Being under the law, He was in duty bound to keep it for Himself.
Answer: In answer to this it may be said that Christ, though possessing a human nature, was yet a divine person, and as such was not subject to the law in its federal aspect, the law as the condition of life in the covenant of works. As the last Adam, however, He took the place of the first. The first Adam was by nature under the law of God, and the keeping of it as such gave him no claim to a reward. It was only when graciously entered into a covenant with him and promised him life in the way of obedience, that the keeping of the law was made the condition of obtaining eternal life for himself and for his descendants. And when Christ voluntarily entered the federal relationship as the last Adam, the keeping of the law naturally acquired the same significance for Him and for those whom the Father had given Him. (380-381)
2.) Objection: God demands, or can demand, only one of two things of the sinner: either obedience to the law, or subjection to the penalty, but not both.
Answer: If the law is obeyed, the penalty cannot be inflicted; and if the penalty is borne, nothing further can be demanded. There is some confusion here, however, which results in misunderstanding. This “either…or” applied to the case of Adam before the fall, but ceased to apply the moment he sinned and thus entered the penal relationship of the law. God continued to demand obedience of man, but in addition to that required of his that he pay the penalty for past transgression. Meeting this double requirement was the only way of life after sin entered the world. If Christ had merely obeyed the law and had not also paid the penalty He would not have won the title to eternal life for sinners; AND IF He had merely paid the penalty, without meeting the original demands of the law, He would have left man in the position of Adam before the fall, still confronted with the task of obtaining eternal life in the way of obedience. By His active obedience, however, He carried His people beyond that point and gave them a claim to everlasting life. (381)
Taken from Louis Berkhof’s Systematic Theology.