Believing for the sake of Christ !
Phil 1:29
29 For to you it has been granted on behalf of Christ, not only to believe in Him, but also to suffer for His sake,
All those who shall believe on Christ as a result of Salvation, it is because it has been granted/given to them to do so. The reason why it has been so granted , was because of His Blood Shed for them, and because its being granted to Him, in other words, He prays that for those He died for to be be made believers through the Word of His chosen messengers Jn 17:19-20
As usual, you have it backwards.
Salvation is the result of belief
John 3:14–15 (KJV 1900) — 14 And as Moses lifted up the serpent in the wilderness, even so must the Son of man be lifted up: 15 That whosoever believeth in him should not perish, but have eternal life.
John 3:16 (KJV 1900) — 16 For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
John 3:36 (KJV 1900) — 36 He that believeth on the Son hath everlasting life: and he that believeth not the Son shall not see life; but the wrath of God abideth on him.
John 5:24 (KJV 1900) — 24 Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.
John 5:25 (KJV 1900) — 25 Verily, verily, I say unto you, The hour is coming, and now is, when the dead shall hear the voice of the Son of God: and they that hear shall live.
John 6:40 (KJV 1900) — 40 And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day.
John 6:47 (KJV 1900) — 47 Verily, verily, I say unto you, He that believeth on me hath everlasting life.
Acts 16:31 (KJV 1900) — 31 And they said, Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house.
John 20:31 (KJV 1900) — 31 But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.
Romans 10:9 (KJV 1900) — 9 That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved.
Romans 10:10 (KJV 1900) — 10 For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation.
Romans 10:13 (KJV 1900) — 13 For whosoever shall call upon the name of the Lord shall be saved.
etc