By this we can be sure that we know Him -- if we practice obedience to His commands.

but whoever practices obedience to His message really has a perfect love of God in his heart. By this we can be sure that we are in union with Him:

Little children, it is the last hour, just as you have heard that Antichrist is Coming, and already many Antichrists have appeared; so we may be sure that 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;

From that we can be sure that we are on the side of the truth, and satisfy our consciences in God's sight,

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.

By the fact that He has given us a portion of His Spirit we can be sure that we remain in union with Him and He in union with us.

This is how we can be sure that we love the children of God, by continuing to love God and to obey His commands.

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 since we are sure that he hears all our prayers, we are sure that the petitions we presented to him, will be answered.