John Calvin, Commentary on Colossians 2:9,10
Further, when he says that the fullness of the Godhead dwells in Christ, he means simply, that God is wholly found in him, so that he who is not contented with Christ alone, desires something better and more excellent than God. The sum is this, that God has manifested himself to us fully and perfectly in Christ.
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And ye are complete in him. He adds, that this perfect essence of Deity, which is in Christ, is profitable to us in this respect, that we are also perfect in him. �As to God�s dwelling wholly in Christ, it is in order that we, having obtained him, may posses in him an entire perfection.� Those, therefore, who do not rest satisfied with Christ alone, do injury to God in two ways, for besides detracting from the glory of God, by desiring something above his perfection, they are also ungrateful, inasmuch as they seek elsewhere what they already have in Christ. Paul, however, does not mean that the perfection of Christ is transfused into us, but that there are in him resources from which we may be filled, that nothing may be wanting to us.