I feel "holy" is not a good word in this context - it tends to carry a negative sense if someone presents himself as holy. Others may regard him as holy.
"..to present himself to God as righteous and as a model of good works to his fellow men."
"Everyone must have a strong understanding of the doctrine..."
Do not use "one" and "him" together. "A believer should be able to defend his beliefs, and with sincere speech and sound reasoning..."