"The aim of our charge is love that issues from a pure heart and a good conscience and a sincere faith" (1 Timothy 1:5, cf. 2 Tim. 1:5, ESV).
"Therefore, brothers, be all the more diligent to make your calling and election sure, for if you practice these qualities you will never fall" (2 Peter 1:10, ESV).
God does not require perfect faith, but sincere faith. It is the object of the faith that saves, not the amount of it. We don't have faith in faith per se, but faith in Christ. You can be sincerely wrong too. You can be 100 percent sure and still go to hell because the faith was misguided. God wants no feigned, pretended, pseudo-faith but an honest faith in Christ. Muslims can be 100 percent sure (by dying in a jihad) and still go to hell because they are misguided. Like Romans 10:2 says the Jews had a zeal for God, but not according to knowledge. In other words, you can have a lot of faith in the wrong thing and be lost. Our faith gets strengthened by trial and tribulation as our faith is tested by God since it is more valuable than silver or gold. Bear in mind that we are saved through the instrumental means of faith (but in Romanism the instrumental means are the sacraments, i.e., baptism, etc.), but our reward is based on our works (Rom. 2:6), not our faith! Faith and faithfulness can be distinguished, but not separated (they are the same Hebrew word in Habakkuk 2:4).
Now, I have had no heavenly vision, divine visitation, dream, heard voices, have had any divine revelation, or existential experience whatsoever to my knowledge, yet I know I am saved. This begs the question: How can I be so presumptuous, as it were, and be so positive without any doubts whatsoever, being convinced 100 percent? After all it is our duty to be sure and find assurance because of the command in 2 Peter 1:10; however, it is not just to satisfy idle curiosity! Without assurance, you will be stunted and paralyzed in your walk and remain an infant in Christ.
The reason most believers doubt (and assurance is not an automatic fruit nor of the essence of salvation, according to The Westminster Confession of Faith, ca 1646, but only for the benefit of its well-being), and this is because they confuse works and faith or fact and feeling. They may also just be ignorant of the Word or even fail to take God at His Word in light of the gospel message! Once they get a biblical view of the subject of assurance they may be reassured. It is not the evangelist's job to give or grant assurance or to certify salvation--that is the domain of the Holy Spirit.
It is because I know the Scriptures and they are a part of my soul and spirit and I know they are dependable and reliable for faith, as the psalmist declared in Psalm 119 so many times. God wants us to be sure and to know, but He wants us to have faith and not be dependent on experience which is so subjective. We can have reassurance in many ways to increase our faith though. No one has the same experience and there is no common ground for faith and fellowship. "For without faith it is impossible to please God." Thomas was gently rebuked by Jesus because he doubted and he had to see Jesus believe (believing is seeing, not seeing is believing!) because Jesus said that "blessed are those who have not seen [or heard] and yet believed [they have more faith]."
1 John 5:13 says that the reason he is writing is so that we can know that we are saved--he doesn't mention any type of O.B.E. or out-of-body experience, near-death experience, hearing of heavenly voices, visits of angels, visions, or dreams, but quotes the Bible. We are to rely on God's plain and simple Word, the way a child would. We need childlike, but not childish faith! Take Him at His Word! This is known as having "spiritual birth certificates." There are other reasons I know I am saved, besides relying on verses like John 6:37 ("He who comes to Me I will in no wise cast out") and John 1:12 ("As many as received Him gave He the right to be the children of God, even them that believe on His name"). I know that Christ died in my stead and rose again on my behalf! God has more to lose than I do if I don't get saved, because His Word says so: "God says it in His Word, I believe it in my heart, that settles it in my mind." I know what God says and believe Him (as 2 Tim. 1:12 says, "I know whom I have believed, and am persuaded that He is able..."). As Michael Faraday said, "I'm not resting on conjecture, but certainties."
There are other factors that reassure me: The way God is changing my life and making me more in the image of Christ; the way God speaks to me through the Bible and many other ways; and especially in fellowship and witness the way the "Spirit bears witness with our spirit that we are the sons of God" (cf. Rom. 8:16). An obvious sign is that I live an abundant, complete, and fulfilled life in Christ that has meaning and purpose and is very rewarding and inspiring, producing fruit. In short, my life has not only been changed, but exchanged, surrendered, renewed, and transformed, and I have not just turned over a new leaf, but my mind, will, and emotions have experienced a complete and total about-face, a complete turnaround, an overhaul of my soul and spirit, a 180-degree turn! It's not that God made me "good," but He gave me life and made me "alive!" However, no matter my experience, I like the refrain:
"My hope is built on nothing less
Than Jesus' blood and righteousness;
I dare not trust the sweetest frame,
But wholly lean on Jesus' name." (Edward Mote)
I know that I have "the Spirit!" I have tested the spirit like it says in 2 Cor. 13:5 (ESV): " ... Test yourselves. Or you not realize this about yourselves, that Jesus Christ is in you? ...." We are to examine ourselves and I have done this to "test the spirit." In other words, we are to examine our own fruit and be fruit inspectors (no fruit--no faith!). Search your own heart--do you love Jesus?
Only God can inspire me as He does and open my eyes to the Word as He does, and guide my life providentially as He does (like in meeting my daily needs)--there is no mistake God is at work in me--anyone that has known my life story could testify to this--my life is a miracle! I have witnessed so much in my life and been the recipient of so much grace that, if I were to ask God for a sign or more evidence, He would simply say, "My grace is sufficient for thee!" I was indeed saved long before I became fully convinced because you don't have to know you're saved to be saved! We are all "works in progress" as it is commonly said. Like Paul says in Philippians 2:13 (ESV) to conclude: "For it is God who works in you, both to will and to work for his good pleasure." Soli Deo Gloria!