But whoever keeps his word, God's love has most assuredly been perfected in him. This is how we know that we are in him:
hereby know we that we are in him: The word "hereby" (or "in this") appears again as it does in verse 3. Here it introduces another test of validity. "We know" experientially "that we are in him." Again, I should emphasize this point: in these days when people seem to be so unsure of their relationship with God and the certainty of their salvation, the words of the apostle John ring down the corridors of time with great assurance and real hope. We can know that we are in Him. To be "in God" is equivalent to being "in Christ," for "God was in Christ, reconciling the world unto himself" (2 Corinthians 5:19). We enter Christ by obedience to His commandments (Acts 10:48; Galatians 3:27; Romans 6:3-4), and our relationship with God in Christ is maintained through continued observance of His commandments. The reoccurring theme of 1 John is fellowship with God. To be "in Christ" is to be in fellowship with God. This fellowship is continued only as long as we walk "in Him" in careful compliance with His will.