'Sure' in the Bible

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

But whoever continually keeps his commandments is the kind of person in whom God's love has truly been perfected. This is how we can be sure that we are in union with God:

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.

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 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.