I am an atheist. I was asked why I thought I received the Holy Spirit when I was a Christian. That was my answer. I don’t believe it but the Bible does promise the Holy Spirit at baptism.
My first thought was that you were being intentionally disingenuous, but maybe you just misheard me. I didn't ask why you thought you had received the Holy Spirit. I asked you how you knew you had received the Holy Spirit, because I took you at your word.
I personally do not believe you received the Holy Spirit. I think there are many who go to church who have yet to receive Him. I think it is why they fall away.
I don't think it is possible to receive Gods Spirit and to then not believe He exists. I think it is possible to get angry and ignore Him or to not progress spiritually past an infant, but to actually meet Him and have intimate relations with Him and to then say you do not believe He exists would be impossible.
To have the seed of the gospel planted but not receive the Spirit because it is snatched away - that I do believe is possible.