Can a muslim man marry a woman who believes in one god but is spiritual and does not believe in religion?
The majority opinion is that Muslims are not allowed to marry non-muslims, monotheists or otherwise, unless they follow a known and attested divine revelation. This is based on the literal interpretation of the following verse:
Quran 5:5 ... And [lawful in marriage are] chaste women from among the believers and chaste women from among those who were given the Scripture before you ...
Classical scholars have used the above to establish prohibition of marrying Zoroastrians, Sabeans and Hanifs etc. even though they are monotheists, the same would extend to a Spiritual Monotheist.
يحرم نكاح من لا كتاب لها كوثنية ومجوسية وتحل كتابية لكن تكره حربية وكذا ذمية على الصحيح والكتابية يهودية أو نصرانية لا متمسكة بالزبور وغيره
Forbidden for marriage those without a Book, such as the idolators and magians. It is permissible to marry people of the Book, but it is makruh (disliked) when they are from a nation with which the Muslims are at war, and it is correct when they are Dhimmis (subjects of a Muslim state). The People of the Book are Jews and Christians, but not those only based on the Zabur (Psalms) etc.
وأما المجوس فالعلماء مجمعون ـ إلا من شَذّ منهم ـ على أن ذبائحهم لا تؤكل ولا يتزوّج منهم؛ لأنهم ليسوا أهل كتاب على المشهور عند العلماء.
Regarding Magians, its the consensus of the ulema, except for a group which dissented, that their slaughtered will not be eaten nor will they be married, because the majority view of the scholars is that they are not people of the book.
وٱختلفوا في نكاح نساءِ المجوس؛ فمنع مالكٌ والشافعيّ وأبو حنيفة والأُوزاعيُّ وإسحاقُ من ذلك. وقال ٱبن حَنْبل: لا يعجبني.
Regarding marrying Magian women, there is a difference of opinion. Imam Malik, Imam Shafi, Imam Abu Hanifa, Awzari and Ishaq have proclaimed its prohibition. Imam Ahmed bin Hanbal said: I don't like it.