Since you said please :) Al Maari indeed criticized religions during most of his life, which is the main reason why, in most references, you are more likely to read about his skeptic beliefs. So, and before I go on, I'm amazed to read some replies that he only turned to Islam in order to save his life and spare his family. Firstly, I wonder where these people found their references since you'll have to know a lot about Al Maari in order to find what I'm talking about (which I doubt those people do). And secondly, Al Maari simply didn't have any relatives most of his life and the guy never married (huge facepalm then!). Now back to your question. As I said before, not easy to find references for that and one has to be almost a scholar and very familiar with Al Maari's works to be aware of this. You can find some elements towards what I said in his Arabic wikipedia page which quotes some of his verses. For example, in one of his poems, you'll read: الله لا ريبَ فيه Which means: There is no doubt about Allah/God (please use any translator to translate the Arabic wikipedia page and check what I'm telling you here). Or, for example: إذا قومُنا لم يعبدوا الله وحدهُ بنُصْحٍ، فإنَّا منهمُ بُرآءُ Which says: if our people don't worship Allah on his own after we advise them to do so, then we are innocent from their deeds. Or again (this one is from another source than wikipedia): يا طالب الطب من داء تخوفه إن الطبيب الذي أبلاك بالداء (To that who seeks the science of medicine to avoid an illness he is afraid of. The true doctor is the one who is testing you with the illness) Again: أدينًُ بربٍّ واحدٍ وتَجَنُّبٍ قبيحَ المساعي (My religion is one with one God and avoiding seeking what is evil).

I hope that I answered, at least, some of your questions. I learnt about Al Maari's poetry and life in Arabic and therefore, it is difficult for me to give you references in English in such a short time. But you are free to seek knowledge on your own, based on what I stated here :)