"You have the words of eternal life, and we believe and are sure that you are the Holy One of God." -- John 6:68-69
Here we have both faith and assurance. Many a doubting heart desires both, when in fact, they have both.
1) The nature of this faith and assurance. "We believe and are sure." Of what? That our names are written in heaven or that we are the elect of God? No. Peter says to the Lord, "We believe and are sure that Thou art the Christ, the Son of the living God, and that Thou hast the words of eternal life."
The disciples believed this and were sure of it. We too must have this foundation. But is this faith and assurance? We may not be sure about much else, but if we believe these truths and are sure of them, then we have a sufficient foundation to build on. It is foundational to the experiential reality of faith and to the feeling or sensibility of assurance. How we should bless the Spirit of God for revealing Christ in His Word, and revealing Him in our hearts according to the Word.
2) It was for a lack of this faith and assurance that "many disciples went back and walked no more with Him." v. 66
It is by believing and being sure that Jesus is the Christ, the Son of the living God, and that He has the words of eternal life that causes any sinner to follow Him, to cleave to Him, to call upon Him, to hope in Him and to expect all salvation from Him.
This may seem like little faith and low-rate assurance, yet the Lord honors it. He pronounces Peter blessed for it in Matthew 16:17. Peter doesn't say, "I believe and am sure that I am a child of God and that my sins are forgiven. But he says, "Thou art the Christ, the Son of the living God." Matthew 16:16
If you really believe these truths and are sure of them, then you have a revelation from God (Matt. 16:17), even while you are doubting. Our very salvation depends on these truths and all our comfort results from the belief and the assurance of it.
"We believe and are sure."
- John Green