Was the bible designed to be understood solely by believers? Are atheists incapable of understanding the bible's intended message?

The fact of the matter, Yes, only when the Lord Jesus Christ opens your Spiritual eyes and Spiritual ears and takes you out of the kingdom of Satan and into the kingdom of the Lord Jesus Christ then your Spiritual eyes and Spiritual ears will be opened to the truth of God word the Bible only, nothing else.

Yes it's true that over 3 quarters of the bible is written to BELIEVERS whether it was the to the Israelites living under the law of Moses in the old testament and partly in the 4 gospels (Joshua 8:31,Mat 5:17-18, Luke 16:16 & John 1:17) or to the christian churches living under grace not the law (Rom 6:14)

But there is ONE book in the new testament God has WRITTEN directly to unbelievers whether they are atheists or religious people and that book is the book of John.

(John 20:31) But these things are WRITTEN, that ye MIGHT BELIEVE that Jesus Christ, is the Son of God; and that BELIEVING ye might have LIFE through his name.

There is NO other book in the N.T that contains a verse like that.

When John later wrote the Ist, 2nd and 3rd books he was writing to BELIEVERS (little children) who KNEW Jesus is the Christ (1John 2:1& 21)

(1John 5:13) 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.

I do believe the book of John was designed by God to tell unbelievers EVERYTHING they NEED to know about who Jesus Christ is (John 1:1) and by believing on him they will have everlasting life (John 6:47)

Of course it's up to a unbeliever to become a believer by reading the book of John and seeking salvation in Jesus Christ by reading other books written to the church on Christ's death, burial and resurrection for their sins and salvation by grace (1Cor 15:1-4 & Eph 2:8-9)

It's a christian's job to preach the word of God and share the gospel with unbelievers when the occasion arises (2Cor 5:18 & 2Tim 4:2)

(Rom 10:17) FAITH cometh... by HEARING the word of God.

some atheists are very smart and some are very crafty, anyone who is a wordy smythe can forge lies from truth if they intend to deceive people

i think thats whats happening, a few smart men build lies that gullible atheists beleive in and the more active spread lies for truth

Was the bible designed to be understood solely by believers?

- It was designed to be "believed" by the ignorant.

Are atheists incapable of understanding the bible's intended message?

- No, we understand it, stupidity is the norm.

"Was the bible designed to be understood for those that WANT it to be true?"

Yes.

Without the Holy Spirit who dwells in all believers, the understanding of the bible is very hard