1 John 4:13

Hereby know we that we dwell in him, and he in us, because he hath given us of his Spirit.

Hereby we know that we dwell in God and He dwells in us because He has given us of His Spirit.
This mutual indwelling takes place in loving one another, by which God dwells in believers and His love is perfected in them.
God's love was shown in sending His only begotten Son into the world that we might live through Him, not because we loved God but because He loved us and sent His Son to be the propitiation for our sins.
Those so loved ought to love one another, not in word or tongue only but in deed and in truth, laying down life for the brethren and helping a needy brother from this world's goods, and love for the unseen God is proved by love for the brother seen.
Dwelling in love is dwelling in God and God in the believer, and this makes love perfect so that believers have boldness without fear in the day of judgment, because as He is so are they in this world.

Spirit Proves Mutual Indwelling

Hereby believers know that they dwell in God and God dwells in them, because He has given them of His Spirit. The gift of the Holy Spirit is the evidence of that mutual indwelling. This knowing is inseparable from loving one another, for in that love God dwells in believers and His love is perfected in them. That love is of God, and everyone who loves is born of God and knows God, while one who does not love does not know God, for God is love. One who does not love his brother cannot truly love God and will stand condemned. The evidence of the Spirit's indwelling is therefore Christlike love with unity and peace among brethren.

That indwelling comes from God's love in sending His only begotten Son into the world so that people might live through Him. It was not because people loved God but because He loved them and sent His Son to be the propitiation for sins and to save sinners. In that sending divinity was joined with humanity so that humanity can share the divine nature and stand with God. So those so loved ought to love one another. That love must be in deed and in truth, not in word or tongue only, following Christ laying down His life, laying down life for the brethren and helping a needy brother from this world's goods.

The indwelling means Christ lives and works in those united with Him in sanctified association and cooperation with Him. Power is provided in Him to reach Christian perfection and to represent Him in lovingkindness of heart, judgment, and righteousness to all. It is gained by keeping Jesus as pattern and by beholding Him and appreciating the virtues of His character, which changes the beholder from glory to glory into His likeness and makes one with the Master in spirit. The Spirit makes Christ's followers complete in Him and is revealed in a life that reflects Christ in upright dealings with no dishonesty. Their good fruit gives evidence of the heart's condition. With Jesus dwelling in their hearts believers think the thoughts of God and do the works of God, and those who have accepted present truth prove that indwelling by being sanctified through the truth.

The gift of the Spirit is received by opened, contrite hearts with living faith, not mere nominal faith or legal religion. It is a faith that appropriates the richest gifts of God, pleads for the gift in order to give to others, and receives of Christ's fullness grace for grace as sons of God. Knowing that God dwells in believers and they in Him because He has given His Spirit is the work God designs His message to perform in the heart and mind of every person. The perpetual life of the church is to love God supremely and to love others as self. Where there is little love for God or man, God gives His messengers what the people need, and those who receive the message are greatly blessed, seeing the bright rays of the Sun of Righteousness with life and hope springing up in their hearts as they behold Christ, hear His everlasting assurance, and apply the blood of the spotless Lamb to their own hearts.

Love Perfected in Believers

The mutual indwelling is known by the gift of the Spirit, and it takes place in loving one another, for when believers love one another God dwells in them and His love is perfected in them. Love for God cannot be separated from Christlike love for fellow believers, and brotherly love for those around us is necessary for the Holy Spirit to work upon hearts and minds. Love is of God, and everyone who loves is born of God and knows God, while one who does not love does not know God and cannot truly love God, and will stand condemned, for God is love.

God's love was manifested in sending His only begotten Son into the world so that people might live through Him, not because people loved God but because He loved them and sent His Son to be the propitiation for sins. The Father's sending of the Son as Saviour of the world is seen and testified, Jesus is confessed as the Son of God, and God's love is known and believed. Those so loved ought to love one another, and love for God unseen is proved by love for the brother seen. Love is to be shown to those without the truth, but by sacred obligation especially to those in the household of faith, by helping and blessing them and jealously guarding every brother and sister, which is working in harmony with Jesus Christ, walking in perfect harmony and glorifying Him, and it gives a faith that purifies the soul.

The verse means love must be in deed and in truth, not in word or tongue only, following Christ laying down His life, laying down life for the brethren and helping a needy brother from this world's goods. The evidence of the Spirit's indwelling is such Christlike love with unity and peace among brethren. No one has seen God, yet dwelling in love is dwelling in God and God in the believer, and this makes love perfect so believers have boldness without fear in the day of judgment, because as He is so are they in this world.