Having confidence and not doubting ones salvation is assurance. This faith and assurance comes by hearing the Word of God.
Ephesians 002:008 For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: 002:009 Not of works, lest any man should boast. John 001:012 But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name: Philippians 004:009 Those things, which ye have both learned, and received, and heard, and seen in me, do: and the God of peace shall be with you. Romans 010:017 So then faith cometh by hearing, and hearing by the word of God.
1 John 5:011 And this is the record, that God hath given to us eternal life, and this life is in his Son. 5:012 He that hath the Son hath life; and he that hath not the Son of God hath not life. 5:013 These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.
Romans 008:001 There is therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit. 008:002 For the law of the Spirit of life in Christ Jesus hath made me free from the law of sin and death. 008:003 For what the law could not do, in that it was weak through the flesh, God s ending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh: 008:004 That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit. 008:005 For they that are after the flesh do mind the things of the flesh; but they that are after the Spirit the things of the Spirit. 008:006 For to be carnally minded is death; but to be spiritually minded is life and peace. 008:007 Because the carnal mind is enmity against God: for it is not subject to the law of God, neither indeed can be. 008:008 So then they that are in the flesh cannot please God. 008:009 But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his. 008:010 And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness. 008:011 But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you. 008:012 Therefore, brethren, we are debtors, not to the flesh, to live after the flesh. 008:013 For if ye live after the flesh, ye shall die: but if ye through the Spirit do mortify the deeds of the body, ye shall live. 008:014 For as many as are led by the Spirit of God, they are the sons of God. 008:015 For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father. 008:016 The Spirit itself beareth witness with our spirit, that we are the children of God: 008:017 And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together. 008:018 For I reckon that the sufferings of this present time are not worthy to be compared with the glory which shall be revealed in us. 008:019 For the earnest expectation of the creature waiteth for the manifestation of the sons of God. 008:020 For the creature was made subject to vanity, not willingly, but by reason of him who hath subjected the same in hope, 008:021 Because the creature itself also shall be delivered from the bondage of corruption into the glorious liberty of the children of God. 008:022 For we know that the whole creation groaneth and travaileth in pain together until now. 008:023 And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body. 008:024 For we are saved by hope: but hope that is seen is not hope: for what a man seeth, why doth he yet hope for? 008:025 But if we hope for that we see not, then do we with patience wait for it. 008:026 Likewise the Spirit also helpeth our infirmities: for we know not what we should pray for as we ought: but the Spirit itself maketh intercession for us with groanings which cannot be uttered. 008:027 And he that searcheth the hearts knoweth what is the mind of the Spirit, because he maketh intercession for the saints according to the will of God. 008:028 And we know that all things work together for good to them that love God, to them who are the called according to his purpose. 008:029 For whom he did foreknow, he also did predestinate to be conformed to the image of his Son, that he might be the firstborn among many brethren. 008:030 Moreover whom he did predestinate, them he also called: and whom he called, them he also justified: and whom he justified, them he also glorified.