"Beloved, although I was very eager to write to you about our common salvation, I found it necessary to write appealing to you to contend for the faith that was once for all delivered to the saints. For certain people have crept in unnoticed who long ago were designated for this condemnation, ungodly people, who pervert the grace of our God into sensuality and deny our only Master and Lord, Jesus Christ." Jude 3-4
Suppose I have a friend Jill who told me she has a sister. I believe my friend since she has never lied to me. However, another friend of mine, Erin, insists that Jill does not have a sister. Since I have reason to believe that Jill is telling the truth, I have to disagree with Erin. While I respect that Erin has her own reason(s) to believe otherwise, I cannot - with honesty and integrity - agree with her.
The bottom line is, Jill either has a sister or she does not. Whichever it is, my friend Erin and I cannot both be right. There are various ways to find out and verify the truth. At the end of the day, the truth exists independently from what we believe. Whether Jill has a sister or not does not depend on what we believe.
Perhaps one could argue that one does not believe a God exists to begin with. I respect that. However, s/he has other questions to answer to. Like how does one explain immaterial entities like love, goodness, and purpose?