We know

1 John 5:14-21
if we ask anything according to his will, he hears us. And if we know that he hears us—whatever we ask—we know that we have what we asked of him.
All wrongdoing is sin,and there is sin that does not lead to death. If you see any brother or sister commit a sin that does not lead to death, you should pray and God will give them life.
We know that anyone born of God does not continue to sin; the One who was born of God keeps them safe, and the evil one cannot harm them.
We know that we are children of God, and that the whole world is under the control of the evil one.
We know also that the Son of God has come and has given us understanding, so that we may know him who is true.
The false teachers loves to talk about 'knowledge', John makes his own list of thing 'we know' for a certainty.

• We know that Christ himself keeps his followers from continuing in the old sinful way of life.
• We know we belong to God in an alien world.
• We know that through the Son of God we know God himself, and share his eternal life.

Having set out the certainties. John urges his readers in closing: 'Do not abandon the real for the illusory(虛幻)'
Why?
When we live in Him, we know God's will so we pray God will listen.
Pray for the brothers wrongdoing, it is sin but which are not lead to death.
Because we know these three truth, we have confidence to face the false teaching. Hold on the truth Jesus have given us.
How?
I know I belong to the Lord, I know I live in God and God is in me, I know through Jesus Christ I share his eternal life.