My twelve year old daughter gave a talk in sacrament meeting that she wrote without any help, and this is the phrase that stuck out the most to me. "Everyone has a testimony, they just haven't found it yet."
How absolutely profound.
When I was being taught by the missionaries and I learned about the pre-existence, I immediately recognized it. Even though I had never heard the concept before, it was what I call a "heart memory". I knew that it was true.
To be sure, not everyone will hear it in the same way I did. Many will hear the gospel and not recognize it.
But it stands to reason, that if we lived with Father in Heaven before our existence on earth, then we all carry the truth within us even if the veil of this life obscures it. We may not be searching for it. We may not recognize it when we stumble across it. We may deny its existence. It may be invisible to us even if we are searching.
But we all have a testimony, even if we haven't found it yet.