To me it is not a question of certainty, but one of a living and very real hope. If we doubt at every turn, where is the faith? Paul called this hope we have like a race.
1Cor 9:24 Do you not know that those who run in a race all run, but one receives the prize? Run in such a way that you may obtain it.
We should take on the race with all we've got, and strive like Jesus said to enter the "narrow gate."
Luke 13:23 Then one said to Him, "Lord, are there few who are saved?"
And He said to them,
v. 24 "Strive to enter through the narrow gate, for many, I say to you, will seek to enter and will not be able.
The Spirit of Christ indwelling should become apparent to us. If your perspective on life is being changed from world mindedness, if your attitude is changed towards mercy and kindness to others, you should know/feel that... you should sense a difference in yourself and over time as you look back you can see how you have changed. It is not an arrogant certainty, because that could even fuel our carnality... but it is very real, a living hope in Christ.
Heb 6:17 Thus God, determining to show more abundantly to the heirs of promise the immutability of His counsel, confirmed it by an oath,
v. 18 that by two immutable things, in which it is impossible for God to lie, we might have strong consolation, who have fled for refuge to lay hold of the hope set before us.
v. 19 This hope we have as an anchor of the soul, both sure and steadfast, and which enters the Presence behind the veil,
v. 10 For God is not unjust to forget your work and labor of love which you have shown toward His name, in that you have ministered to the saints, and do minister.
v. 11 And we desire that each one of you show the same diligence to the full assurance of hope until the end,
mercy, peace and love
Kat