"In reality the growth of the individual does not take place except in the fellowship of the church as the fellowship of the Spirit.  Believers have never existed as independent units.  In God's eternal counsel they were chosen in Christ; in the accomplishment of their redemption they were in Christ; in the application of redemption they are ushered into the fellowship of Christ. And sanctification itself is a process that moves to a consummation which will not be realized for the individual unit until the whole body of Christ is complete and presented in its totality faultless and without blemish.  This points up the necessity of cultivating and promoting the sanctification of the whole body, and the practical implications for responsibility, privilege and opportunity become apparent.

If the individual is indifferent to the sanctification of others, and does not seek to promote their growth in grace, love, faith, knowledge, obedience and holiness, this interferes with his own sanctification in at least two respects: 1.) His lack of concern for others is itself a vice which gnaws at the root of his spiritual growth.  If we are not concerned with or vigilant in respect of the fruit of the Spirit in others, then it is because we do not burn with holy zeal for the honor of Christ himself.  All shortcoming and sin in us dishonour Christ and a believer betrays the coldness of his love to Christ when he fails to bemoan the defects of those who are members of Christ's body.  2.) His indifference to the interests of others means the absence of the ministry which he should have afforded others.  This absence results in the impoverishment of these others to the extent of his failure, and this impoverishment reacts upon himself, because these others are not able to minister to him the full extent of the support, encouragement, instruction, edification, and exhortation which they owe him."

John Murray