There's the certainty, unless you want to try to tell us that the Apostle John lied to us.
1 John 5:9-15 9 If we receive the testimony of men, the testimony of God is greater, for this is the testimony of God that he has borne concerning his Son. 10 Whoever believes in the Son of God has the testimony in himself. Whoever does not believe God has made him a liar, because he has not believed in the testimony that God has borne concerning his Son. 11 And this is the testimony, that God gave us eternal life, and this life is in his Son. 12 Whoever has the Son has life; whoever does not have the Son of God does not have life.
13 I write these things to you who believe in the name of the Son of God that you may know that you have eternal life. 14 And this is the confidence that we have toward him, that if we ask anything according to his will he hears us. 15 And if we know that he hears us in whatever we ask, we know that we have the requests that we have asked of him.
Or perhaps you are going to claim that Jesus lied to us?
John 5:24 Truly, truly, I say to you, whoever hears my word and believes him who sent me has eternal life. He does not come into judgment, but has passed from death to life.
John 10:9 I am the door. If anyone enters by me, he will be saved and will go in and out and find pasture.
John 10:27-29 27 My sheep hear my voice, and I know them, and they follow me. 28 I give them eternal life, and they will never perish, and no one will snatch them out of my hand. 29 My Father, who has given them to me, is greater than all, and no one is able to snatch them out of the Father's hand.
Hebrews 11:1-2 Now faith is the assurance of things hoped for, the conviction of things not seen.
Faith is taking God at his word about spiritual realities.
If someone doesn't believe that what God says is true, they don't have faith in Him.
amen and amen!