Being saved by faith provides ways for a believer to know without a doubt, not having to guess or revert beck to hoping only.
Peter set that issue beyond question.
2 Peter 1:1-11 (KJV)
[SUP]1 [/SUP] Simon Peter, a servant and an apostle of Jesus Christ, to them that have obtained like precious faith with us through the righteousness of God and our Saviour Jesus Christ:
[SUP]2 [/SUP] Grace and peace be multiplied unto you through the knowledge of God, and of Jesus our Lord,
[SUP]3 [/SUP] According as his divine power hath given unto us all things that pertain unto life and godliness, through the knowledge of him that hath called us to glory and virtue:
[SUP]4 [/SUP] Whereby are given unto us exceeding great and precious promises: that by these ye might be partakers of the divine nature, having escaped the corruption that is in the world through lust.
[SUP]5 [/SUP] And beside this, giving all diligence, add to your faith virtue; and to virtue knowledge;
[SUP]6 [/SUP] And to knowledge temperance; and to temperance patience; and to patience godliness;
[SUP]7 [/SUP] And to godliness brotherly kindness; and to brotherly kindness charity.
[SUP]8 [/SUP] For if these things be in you, and abound, they make you that ye shall neither be barren nor unfruitful in the knowledge of our Lord Jesus Christ.
[SUP]9 [/SUP] But he that lacketh these things is blind, and cannot see afar off, and hath forgotten that he was purged from his old sins.
[SUP]10 [/SUP] Wherefore the rather, brethren, give diligence to make your calling and election sure: for if ye do these things, ye shall never fall:
[SUP]11 [/SUP] For so an entrance shall be ministered unto you abundantly into the everlasting kingdom of our Lord and Saviour Jesus Christ.
That list is how the Way was set up for us, who enter into the path by hearing the gospel of Christ, believe it by faith, then follow that Way. It would make no sense for anyone to merely once believe by faith, but abandon faith in the Savior, walking away like Judas Iscariot, then be certain they are still saved. Can anyone show Judas never believed on Jesus? He took part in every facet of that ministry. Judas walked with Christ those years, fell due to failure of proper diligence, and remained fallen. If fallen, get up, resume following in repentance, giving all diligence to the right things. That way we are not among the number Jesus says he "lost" (Jn 17:12). Many others followed, but were not given to Jesus by the Father. Many fell away besides Judas.
If failing in that plan of salvation from the corruption/wickedness of the world, there is no question that the backslider will fall, his election certainly unsure.
Considering that everything God created was "good", Satan was part of that creation of angels, could not have been created already doubting or resisting God, but in fact had to have been approved of God in the beginning of that relationship. He couldn't have rebelled apart from being first on God's side. Satan yielded his righteousness and glory, chose sin after having known God face to face.
As to the OP question, I'm sure of the promises of God because above my ability to change, God rebirthed me. Others in my life noticed the miracle of a changed life before I put it together. I heard the gospel preached powerfully one day, believed it, and immediately was changed. For me that was bewildering for months, but for those witnesses it was all profoundly amazing, seeing a sudden change from unholiness to the righteousness of Christ. Since I was then convinced I was a new creature in Christ, I had confidence all the other promises had to be literally true. But so are the conditions of salvation, adding many virtues to faith. The more added, the more sure I a that the faith is true.
