“God hath sent forth the Spirit of his Son into your hearts.” – Galatians 4:6

Note, that it does not say into your heads or your brains. The Spirit of God doubtless illuminates the intellect and guides the judgement, but this is not the commencement nor the main part of His work. He comes chiefly to the affections, He dwells with the heart, for with the heart man believeth unto righteousness, and “God hath sent forth the Spirit of His Son into your hearts.” Now, the heart is the centre of our being, and therefore doth the Holy Ghost occupy this place of vantage. He comes into the central fortress and universal citadel of our nature, and thus takes possession of the whole. The heart is the vital part; we speak of it as the chief residence of life, and therefore the Holy Ghost enters it, and as the living God dwells in the living heart, taking possession of the very core and marrow of our being. It is from the heart and through the heart that life is diffused. The blood is sent even to the extremities of the body by the pulsings of the heart, and when the Spirit of God takes possession of the affections, He operates upon every power, and faculty, and member of our entire manhood. Out of the heart are the issues of life, and from the affections sanctified by the Holy Ghost all other faculties and powers receive renewal, illumination, sanctification, strengthening, and ultimate perfection.

This wonderful blessing is ours “because we are sons;” and it is fraught with marvellous results. Sonship sealed by the indwelling Spirit brings us peace and joy; it leads to nearness to God and fellowship with Him; it excites trust, love, and vehement desire, and creates in us reverence, obedience, and actual likeness to God. All this, and much more, because the Holy Ghost has come to dwell in us. ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/1435.cfm