We know that the Scriptures were given to us to grant us such assurance. The apostle John writes in 1 John 5:13:(NASB)
" These things I have written to you who believe in the name of the Son of God, in order that you may know that you have eternal life."
It is God's will that all be saved:
2 Peter 3:9 (NASB) "The Lord is not slow about His promise, as some count slowness, but is patient toward you, not wishing for any to perish but for all to come to repentance."
Christ died for every man:
Hebrews 2:9 (NASB) "But we do see Him who has been made for a little while lower than the angels, namely, Jesus, because of the suffering of death crowned with glory and honor, that by the grace of God He might taste death for everyone."
The Gospel was to be preached to every person:
Mark 16:15 (NASB) "And He said to them, " Go into all the world and preach the gospel to all creation."
You may be assured of salvation if you have done the following things:
1.) If you believe in Jesus:
John 8:24 (NASB) "I said therefore to you, that you shall die in your sins; for unless you believe that I am He, you shall die in your sins."
2.) Repent of your sins: Luke 13:3 (NASB) "I tell you, no, but unless you repent, you will all likewise perish."
3.) Confess Christ:
Romans 10:9, 10 (NASB) – "that if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you shall be saved; for with the heart man believes, resulting in righteousness, and with the mouth he confesses, resulting in salvation."
4.)And are Baptized for the remission of sins by immersion:
Acts 2:38 (NASB) "And Peter said to them, "Repent, and let each of you be baptized in the name of Jesus Christ for the forgiveness of your sins; and you shall receive the gift of the Holy Spirit."
Then, your sins are forgiven and you are saved:
1 Peter 3:21 (NASB) "And corresponding to that, baptism now saves you–not the removal of dirt from the flesh, but an appeal to God for a good conscience–through the resurrection of Jesus Christ. ."
And the Lord adds you to His church: Acts 2:47 (NASB)"praising God, and having favor with all the people. And the Lord was adding to their number day by day those who were being saved."
A warning must be issued here: It then becomes possible to lose your salvation: John 15:6(NASB) "If anyone does not abide in Me, he is thrown away as a branch, and dries up; and they gather them, and cast them into the fire, and they are burned."
Paul said some will fall away –
1 Timothy 4:1 "But the Spirit explicitly says that in later times some will fall away from the faith, paying attention to deceitful spirits and doctrines of demons"
Timothy was instructed to pay close attention to himself and his teaching to insure salvation:
1 Timothy 4:16 "Pay close attention to yourself and to your teaching; persevere in these things; for as you do this you will insure salvation both for yourself and for those who hear you."
So how can I be certain about it? The apostle Peter gives us detailed instruction in 2 Peter 1:5-10:
5 "Now for this very reason also, applying all diligence, in your faith supply moral excellence, and in your moral excellence, knowledge;
6 and in your knowledge, self-control, and in your self-control, perseverance, and in your perseverance, godliness;
7 and in your godliness, brotherly kindness, and in your brotherly kindness, love.
8 For if these qualities are yours and are increasing, they render you neither useless nor unfruitful in the true knowledge of our Lord Jesus Christ.
9 For he who lacks these qualities is blind or short-sighted, having forgotten his purification from his former sins.
10 Therefore, brethren, be all the more diligent to make certain about His calling and choosing you; for as long as you practice these things, you will never stumble. . ."
You can be saved, and then you can do one of two things with your salvation. You can lose it, or you can insure it. It's up to you. God has done his part. His word gives assurance. If we do as the Word of God directs there should be no doubts.