This is how we are sure that we have come to know Him: by keeping His commands.

But whoever habitually keeps His word and obeys His precepts [and treasures His message in its entirety], in him the love of God has truly been perfected [it is completed and has reached maturity]. By this we know [for certain] that we are in Him:

Children, it is the last hour. You have learned that 'Antichrist is coming.' Well, but many antichrists have appeared ??which makes us sure it is the last hour.

If you know that He is absolutely righteous, you know [for certain] that everyone who practices righteousness [doing what is right and conforming to God’s will] has been born of Him.

Little children, make sure no one deceives you; the one who practices righteousness is righteous, just as He is righteous;

Thus it is that we may be sure we belong to the truth and reassure ourselves

The person who keeps his commandments abides in God, and God abides in him. This is how we can be sure that he remains in us: he has given us his Spirit.

This is how we may be sure we remain in him and he in us, because he has given us a share in his own Spirit;

This is how we are sure that we love God's children, by loving God and obeying his commands

(Added verse not retained in Sawyer)

Now if we admit the testimony of men, sure the testimony of God is greater: and this is the testimony of God, which He hath testified concerning his Son.

I have written in this way to you who believe in the name of the Son of God, that you may be sure you have life eternal.

This is the [remarkable degree of] confidence which we [as believers are entitled to] have before Him: that if we ask anything according to His will, [that is, consistent with His plan and purpose] He hears us.

And if we know that he listens to our requests, we can be sure that we have what we ask him for.