Scripture, please. This is one of those Protestant theological fantasies. Sanctification is gradual and lasts a lifetime.
[2] Grace to you and peace be accomplished in the knowledge of God and of Christ Jesus our Lord: [3] As all things of his divine power which appertain to life and godliness, are given us, through the knowledge of him who hath called us by his own proper glory and virtue. [4] By whom he hath given us most great and precious promises: that by these you may be made partakers of the divine nature: flying the corruption of that concupiscence which is in the world. [5] And you, employing all care, minister in your faith, virtue; and in virtue, knowledge; [6] And in knowledge, abstinence; and in abstinence, patience; and in patience, godliness; [7] And in godliness, love of brotherhood; and in love of brotherhood, charity. [8] For if these things be with you and abound, they will make you to be neither empty nor unfruitful in the knowledge of our Lord Jesus Christ. [9] For he that hath not these things with him, is blind, and groping, having forgotten that he was purged from his old sins. [10] Wherefore, brethren, labour the more, that by good works you may make sure your calling and election. For doing these things, you shall not sin at any time. (2 Peter 1)Be you therefore perfect, as also your heavenly Father is perfect (Matthew 5:48)
If thou wilt be perfect, go sell what thou hast, and give to the poor, and thou shalt have treasure in heaven: and come follow me (Matthew 19:21)
"You don't become a saint by *acting like one*. You act like one after you become a saint."
"Scripture, please. This is one of those Protestant theological fantasies. Sanctification is gradual and lasts a lifetime."
Speaking for myself, and i think may others, while i uphold that it is faith alone that saves, i do not hold that it is a faith that is alone, nor did classic Protestantism, but that faith that has no holiness and works is not salvific.
The meek shall inherit the earth, and it not vain hearers but "the doers of the law shall be justified," (Rm. 2:13) and Jesus is "the author of eternal salvation unto all them that obey him," (Heb. 5:9) not due to the merit of said works but because such is an attribute of saving faith.
The convicted and contrite sinner comes to the crucified and risen Lord Jesus Christ destitute of any means or merit whereby he may escape his just eternal damnation or merit eternal life, and casts all his faith in the mercy of God in Christ, trusting Him to save him by His blood and righteousness. And having made a basic turn in his heart from darkness to light, and resting in Christ as his savior, the response of such salvific faith is to follow Him as his Lord, according to the light he has. And repent when convicted of not doing so.
Yet while your second statement, that "sanctification is gradual and lasts a lifetime" is correct, and you cannot deny that it is initial sanctification that makes one a saint, and the oft repeated appeal of the Holy Spirit under the New Covenant is for believers to live out practically what they are positionally.
Acts 9:31,32,41; 26:10; Rm. 1:7; 8:27; 12:13; 15:25,26,31; 16:2,15 and more texts refers to believers in general , as saints.
The Romans were told of the salvation and position they had with God by faith before being exhorted to live it out. (Rm. 1-5ff)
The Corinthians were also called to be saints, (1Cor. 1:2; cf. 6:1,2; 14:33; 16:1,15, etc.) with “to be” being not in the Greek where it is used, and after chastising them for their overall immaturity and carnality, they were told told,
And such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus, and by the Spirit of our God. (1 Corinthians 6:11)
Yet in the next book those who did not repent of sins of the flesh were warned of being reprobate, being out of the faith. (2Cor. 12:21; 13:5)
After warning the problematic Galatians not to “bite and devour one another,” Paul exhorted them, “If we live in the Spirit, let us also walk in the Spirit.” (Galatians 5:25)
As per that pattern, the Ephesians were told that they were positionally made to “sit together in heavenly places in Christ Jesus,” (Eph. 2:6) and then exhorted to live it out.
Likewise the Colossians were exhorted, “If ye then be risen with Christ, seek those things which are above, where Christ sitteth on the right hand of God.” (Col 3:1)
And which is a principle which continues, as rather than seeking to gain acceptance of God and salvation, believers are first made “accepted in the Beloved,” (Eph. 1:6) and then, giving with life from a position of strength, they both rightly motivated and enabled to become more like Christ and to do His will. Not in order to gain salvation, but to continue in the faith, and response to so great a salvation through Jesus Christ the Lord, “whose house are we, if we hold fast the confidence and the rejoicing of the hope firm unto the end." (Hebrews 3:6) As such enduring faith is a mark of the elect.
As for knowing you are saved, the Bible , both provides for that, as well as the means by which one will never fall. 1Jn. 5:13, referring to all that he wrote previously, and appealing to spiritual judgment, offers that one can have assurance of eternal life if his faith corresponds to the marks of saving faith delineated therein.
In addition, the text you posted, 2 Peter 1:1-10, calls believers is to add to their faith things which make for maturity, with the promise that if one gives diligence to make his calling and election sure, then he “shall never fall.”
However I have much to go, especially in heart. May all that is within me cry glory, and nothing grieve His Holy Spirit.