You're basing that statement on you're belief, not on anything in scripture.
It is based upon scripture. As several people have tried to point out, to believe requires a new mind and new heart. That is a
a prerequisite to understanding the gospel. A new mind and heart are given only when born again by God, it is not of ourselves.
As for the eunuch, he had to be born again in order to understand and believe because it requires a new mind and heart to do so.
[Luk 1:77-78 KJV] 77 To
give knowledge of salvation unto his people by the remission of their sins,
78
Through the tender mercy of our God; whereby the dayspring from on high hath visited us,
[Heb 10:16 KJV] 16 This [is] the covenant that I will make with them after those days, saith the Lord, I will put my laws into their hearts, and
in their minds will I write them;
[Rom 12:2 KJV]
2 And be not conformed to this world: but
be ye transformed by the renewing of your mind, that ye may prove what [is] that good, and acceptable, and perfect, will of God.
[Tit 3:5 KJV] 5 Not by works of righteousness which we have done, but according to his mercy he saved us, by the washing of regeneration,
and renewing of the Holy Ghost;
A person must have knowledge of something in order to believe it.
One's belief does not save them. Belief is a fruit of the Spirit that comes from being born-again.
[Gal 5:22 KJV] 22
But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness,
faith,
It's a spiritual event that occurs when a person decides to believe the gospel (Eph 1:13).
Being sealed with the Holy Spirit is not the same as being indwelt by the Holy Spirit.
BTW, to "hear" is spiritual hearing, not human hearing; belief is from the Holy Spirit - see Gal 5:22 above.
[Eph 1:13 KJV] 13 In whom ye also [trusted], after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise,