Did you ever think of the awful dishonor done not only to the Spirit of God, but to Christ by the denial of the permanency of His abiding in the believer? If the Spirit could leave, after having taken up His abode in us, it would involve a denial of the work of Christ. His work would have ceased to avail before God.
It cannot be too clearly understood that this indwelling is not because of anything in us, either at the beginning, or at any stage of the Christian life.
From first to last, the Spirit dwells with us because of the unchanging value of the work of Christ. Cease forever to dishonor the value of that work by doubting the presence of this Holy Person. Your feelings, your faithfulness have nothing to do with this basic fact.