First, it is incredible that we are the children of God!
Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not.
Right on the heels of 2:29 where John first introduces the very famous phrase from his gospel “born of Him”, is this burst of praise that a) God’s love is foreign to us; b) God’s love is lavished upon us; c) God’s love makes us His children. We’re fairly (and unfortunately) inoculated to this truth. It is so common to us “that we should be called the sons of God” that we’ve lost the grandeur and phenomenal-ness of this simple truth.
The word “manner” stems from a somewhat obscure Greek word that means “of another world” or “foreign”. It’s what the disciples said of Jesus when He calmed the storm in Mark 4:41 “And they feared exceedingly, and said one to another, What manner of man is this, that even the wind and the sea obey him?” They clearly indicated that what they had witnessed from Jesus was not normal, not of their world. And that is exactly what John is trying to convey here. What manner of love is this?
Second, ongoing sin is an indication that you are not a genuine Christian.
The reason this is so is because: 1) sin’s nature is lawlessness and 2) sin’s origin is of the devil. True Christians are at peace with God (through the sacrificial blood death of Jesus) and are born of God. To war against Him (sin is lawlessness) and join with the devil (sin’s origin) makes no sense and cannot be.
John fires away in rapid succession with his “whosoever” statements:
Whosoever committeth sin transgresseth also the law…(3:4)
Whosoever abideth in him sinneth not: (3:6)
Whosoever is born of God doth not commit sin…(3:9)
There is simply no room for justification of sin here. Of course, the Bible is here conveying an ongoing, continued pattern of repeated sin, no the occasional or infrequent sin.
So, sin is to have hardly any presence in the life of a truly born again, Christ-loving Christian. It’s one of the ways you can know whether you are genuinely one of God’s children.