Shaykh Salih al-Fawzan advises that if someone denies the existence of any Lord, then the caller to Islam should not waste time trying to convince them. Such people are atheists, and it is better to leave them rather than be overly ambitious in calling them.
Some of the western duʿāt say when they are inviting the non-Muslims to Islam and tawḥīd of Allah ﷻ, that the non-Muslims tell them “We do not believe in the existence of a Lord at all..” So what is the correct path in giving daʿwah to these people?
If they do not know of a Lord at all then leave them. Do not be ambitious with those people. Those people are atheists.