If all were one member He means, that God has not acted at random, or without good reason, in assigning different gifts to the members of the body; but because it was necessary that it should be so, for the preservation of the body; for if this symmetry were taken away, there would be utter confusion and derangement. Hence we ought to submit ourselves the more carefully to the providence of God, which has so suitably arranged everything for our common advantage. One member is taken here to mean a mass, that is all of one shape, and not distinguished by any variety; for if God were to fashion our body into a mass of this kind, it would be a useless heap. [1]

Footnotes

1 - "Un amas de chair inutile;" -- "A heap of useless flesh."

And if all were one member - If there were nothing but an eye, an ear, or a limb, there would be no body The idea which this seems intended to illustrate is, that if there was not variety of talent and endowment in the church, the church could not itself exist. If, for example, there were nothing but apostles, or prophets, or teachers; if there were none but those who spoke with tongues or could interpret them, the church could not exist. A variety of talents and attainments in their proper places is as useful as are the various members of the human body.

And if they were all one member,.... As all eye, or all ear, or all hand, or all foot:
where were the body? where would be the body? it would not be a body consisting of such proper and suitable members, as now it is: so if the community of the saints were either all ministers, or all hearers, &c. there would be no body, consisting of different persons, to receive any benefit or usefulness from either; the church of Christ would not be that uniform, useful, and consistent community it is.

where were the body--which, by its very idea, "hath many members" (1-Corinthians 12:12, 1-Corinthians 12:14), [ALFORD].