“By this we know that we abide in Him, and He in us, because He has given us of His Spirit.”
Dwelling in love means that a person is in fellowship with God (4:13-17). John now turns from a warning against believing just any spirit (4:1-6) and the appeal to love one another (4:7-12) to personal application of living life by the power of the Spirit of God (4:13f).
By this we know that we abide in Him,
The believer has assurance that he is in fellowship with the Lord [abides] because of the Spirit whom He gave us. Abiding is synonymous with knowing Christ (2:24-25).
and He in us,
There is a mutual indwelling. We dwell or abide in God and He abides in us. God fellowships with us when we allow Him to control our lives.
because He has given us of His Spirit
The reference here is not to the gift of the indwelling Spirit at the instant we became Christians but rather to the manifestations of the Spirit in our lives. The Greek for “of” in the phrase “of His Spirit” indicates the product of participation in the Spirit’s presence. Love is a grace that flows from the Spirit. The Holy Spirit assures us of our fellowship with God when we love one another.
Love is the authenticating test of the Holy Spirit producing fruit in us.
Spirit-fellowship producing divine love in us is evidence of fellowship with God. The love we manifest to other Christians is an outcome of the gift of the Holy Spirit to us. The Holy Spirit is the source of the believer’s love just as He is the source of our application of truth to experience (3:23-24).
3: 24 “Now he who keeps His commandments abides in Him, and He in him. And by this we know that He abides in us, by the Spirit whom He has given us.”
On the Day of Pentecost God poured out His Spirit on the church and on each individual believer. He made the things of Jesus real to the believer. He made the love of God real to us.
Ro 5: 5 “Now hope does not disappoint, because the love of God has been poured out in our hearts by the Holy Spirit who was given to us.”
The outpouring of love from the Spirit assures us that we habitually abide in fellowship with God.