Peter T. O'brien:
[Christ is all] states in an emphatic way that Christ is "absolutely everything" . . . or "all that matters . . . ."
To claim that Christ is "all" is briefly to reiterate the high Christology that Paul has set forth in [Colossians] 1:15-20: Christ is the center point of both creation and redemption, the one in whom and through whom all things now hold together.
[Christ is all] speaks of Christ being the total concern, preoccupation, and environment of the Christian.
Christ is all, and in all, that is, Christ alone holds, as they say, the prow and the stern - the beginning and the end.
Christ is a Christian's all, his only Lord and Saviour, and all his hope and happiness. And to those who are sanctified, one as well as another and whatever they are in other respects, he is all in all, the Alpha and Omega, the beginning and the end: he is all in all things to them.
The IVP New Testament Commentary:
The concluding formula, Christ is all, and is in all, echoes the confession of [Colossians] 1:15-20 and once again lays claim to Christ's lordship over the new order. In Schweizer's words, "Christ is the measure by which everything is to be defined" (1972:200); he is "all that matters" (Harris 1991:154). This is the firm conviction of the new humanity - those who have already "taken off" the fallen order and "put on" the good intentions of the Creator which they have begun to realize in Christ."
Christ is all, and in all; he is "all" efficiently; he is the first cause of all things, the beginning of the creation of God, the author of the old, and of the new creation, of the regeneration of his people, and of their whole salvation: he is all comprehensively; has all the fullness of the Godhead, all the perfections of deity in him; he is possessed of all spiritual blessings for his people; and has all the promises of the covenant of grace in his hands for them; yea, all fullness of grace dwells in him, in order to be communicated to them: and he is all communicatively; he is their light and life, their wisdom, righteousness, sanctification, and redemption, their food and clothing, their strength and riches, their joy, peace, and comfort, who gives them grace here, and glory hereafter . . . and Christ is "in all;" in all places, being infinite, immense, and incomprehensible, as God, and so is everywhere by his power, upholding all things by it; and in all his churches, by his gracious presence, and in the hearts of all his regenerate ones, of whatsoever nation, state, and condition they be: he is revealed in them, formed within them, and dwells in their hearts by faith; and is all in all to them, exceeding precious, altogether lovely, the chiefest among ten thousands, and whom they esteem above all creatures and things. The Arabic version reads, "Christ is above all, and in all."