Everyone thinks their current beliefs are true.
And it’s part of our nature (an immature part, yes, but definitely innate) to look for only confirmation, and to resist all challenges.
You could deliver the perfect argument, dense with relevant reason leading to inescapable conclusions, and you’d be met with only at best a glazed-over smile and a twinkle in the eye for all your trouble.
Instead, dig first to the very core of the person’s values.
Their beliefs are what account for and prescribe their estimations of value, but values themselves are objective.
People argue over which values are more/most important based on their beliefs.
So be able to say back to the person exactly what they value.
Then dig some more to discover why they believe what they do.
For example, be able to say back, “So you believe the peace and freedom you feel when you consider Jesus dying for your sins is the Holy Spirit confirming that you’re a forgiven child of God?”
And that’s it. Say nothing more than their own values and beliefs back to them.
Following that, a long-term strategy would be to build relationship, earn trust, and let them see what kind of person you are. Don’t even discuss beliefs again until they broach the subject.
And trust me, they will.
That’s when they’ll be open to hear what you believe and why.