The answer to those challenging absolute certainty about the God of Christianity
No one has an excuse not to be absolutely certain about the God of Christianity:
For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. Claiming to be wise, they became fools...
(Romans 1:20-22 ESV)