This question comes up all the time.
It makes me think people (not you) see the spiritual world as some kind of ethereal place that is somewhat real and somewhat unreal coming in and out of existence depending on if I am believing in it.
God has set the condition it takes one act of genuine faith IN the completed work of Christ Jesus.
Justification is a reality, a permanent outcome/result.
If the Judge declares justified, it not my ongoing belief in the status of justified, nor what allowed me to receive the gift of justification that keeps me justified, it is the fact that the Judge has made the declaration.
The past no longer exists. The future does not yet exist. Only the present exists. It makes no sense to me for someone to claim "I was saved, and even though I am unChristlike right now, I will be like Christ one day; so what I AM today, how I live and behave today, is not really significant.
OSAS could be seen as a post hoc rationalisation invented by underperforming Christians at one time to make the Word seem to do what the Holy Spirit was not doing, giving them assurance of present connection to and approval of God despite not presently expressing the love of God. Let's focus on expressing the Spirit of Jesus. It is not growing to Christian maturity to collect the biggest bag of truth claims one can. A small child can produce truth claims and insist they are true and defend them. "That's a dog, not a cat." Christian maturity is growing to more and more behave like Jesus in the world despite pushback. An ability to propose OSAS and to defend it, or to argue against it, indicates nothing about one's Christian maturity, IMO. There are far more meaningful benchmarks we should be using to measure our intimacy with and maturity in Christ.
1 John 2:
3 And hereby we do know that we know him, if we keep his commandments.
4 He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him.
5 But whoso keepeth his word, in him verily is the love of God perfected: hereby know we that we are in him.
6 He that saith he abideth in him ought himself also so to walk, even as he walked.
7 Brethren, I write no new commandment unto you, but an old commandment which ye had from the beginning. The old commandment is the word which ye have heard from the beginning.
8 Again, a new commandment I write unto you, which thing is true in him and in you: because the darkness is past, and the true light now shineth.
9 He that saith he is in the light, and hateth his brother, is in darkness even until now.
10 He that loveth his brother abideth in the light, and there is none occasion of stumbling in him.
11 But he that hateth his brother is in darkness, and walketh in darkness, and knoweth not whither he goeth, because that darkness hath blinded his eyes.
28 And now, little children, abide in him; that, when he shall appear, we may have confidence, and not be ashamed before him at his coming.
29 If ye know that he is righteous, ye know that every one that doeth righteousness is born of him.
1 john 3:2 Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is.
3 And every man that hath this hope in him purifieth himself, even as he is pure.
4 Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.
5 And ye know that he was manifested to take away our sins; and in him is no sin.
6 Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him.
7 Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous.
8 He that committeth sin is of the devil; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
9 Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God.
10 In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother.
11 For this is the message that ye heard from the beginning, that we should love one another.
12 Not as Cain, who was of that wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous.
13 Marvel not, my brethren, if the world hate you.
14 We know that we have passed from death unto life, because we love the brethren. He that loveth not his brother abideth in death.
15 Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him.
16 Hereby perceive we the love of God, because he laid down his life for us: and we ought to lay down our lives for the brethren.
17 But whoso hath this world's good, and seeth his brother have need, and shutteth up his bowels of compassion from him, how dwelleth the love of God in him?
18 My little children, let us not love in word, neither in tongue; but in deed and in truth.
19 And hereby we know that we are of the truth, and shall assure our hearts before him.
20 For if our heart condemn us, God is greater than our heart, and knoweth all things.
21 Beloved, if our heart condemn us not, then have we confidence toward God.
23 And this is his commandment, That we should believe on the name of his Son Jesus Christ, and love one another, as he gave us commandment.
24 And he that keepeth his commandments dwelleth in him, and he in him. And hereby we know that he abideth in us, by the Spirit which he hath given us.
4:7 Beloved, let us love one another: for love is of God; and every one that loveth is born of God, and knoweth God.
8 He that loveth not knoweth not God; for God is love.
9 In this was manifested the love of God toward us, because that God sent his only begotten Son into the world, that we might live through him.
10 Herein is love, not that we loved God, but that he loved us, and sent his Son to be the propitiation for our sins.
11 Beloved, if God so loved us, we ought also to love one another.
12 No man hath seen God at any time. If we love one another, God dwelleth in us, and his love is perfected in us.
13 Hereby know we that we dwell in him, and he in us, because he hath given us of his Spirit.
16 And we have known and believed the love that God hath to us. God is love; and he that dwelleth in love dwelleth in God, and God in him.
17 Herein is our love made perfect, that we may have boldness in the day of judgment: because as he is, so are we in this world.
18 There is no fear in love; but perfect love casteth out fear: because fear hath torment. He that feareth is not made perfect in love.
19 We love him, because he first loved us.
20 If a man say, I love God, and hateth his brother, he is a liar: for he that loveth not his brother whom he hath seen, how can he love God whom he hath not seen?
21 And this commandment have we from him, That he who loveth God love his brother also.
5:1 Whosoever believeth that Jesus is the Christ is born of God: and every one that loveth him that begat loveth him also that is begotten of him.
2 By this we know that we love the children of God, when we love God, and keep his commandments.
3 For this is the love of God, that we keep his commandments: and his commandments are not grievous.
13 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.