:salamext:
Sister, when atheists say you don't need a religion to understand God - then you ask them what their definition of religion is. In Islam, we need to understand who Allah is through revelation, through His Messengers' and from them we are able to understand who Allah is, what He wants from us, and what will help us reach our destination of Allah's pleasure and reward.
By saying that you can understand God without religion, then you're saying that you can understand God without guidance from Allah himself.
They may argue that you can, but its likely that they don't have a perfect system which covers all the ways of life which Islam teaches through one example (in our case, Muhammad (sal Allah alaihi wasalam)) - for them, they'll wander around blindly, not really being sure they're upon the truth since they will have to find their own answers for wrong and right, and they'll continuously live a life of contradiction on these issues since they won't be certain of what they believe, wandering to the left and sometimes to the right. This then isn't really a religion, but a selection of varying choices which the person keeps differing in. Whereas for a religion to be true, it has firm principles which the believer follows, a light so he can be sure what is right and what is wrong without contradiction and error.
That's why you need 'organized' religion which will be organized (not 'disorganised) so you know what is true, what is right, from falsehood and error. Otherwise you're just walking around blindly without a firm set truth. So you don't truly understand God, nor can you get close to Him, accept with a true Messenger who you don't doubt in his claim of Prophethood.
And Allah knows best.