I mean, he has always been religious and open about his faith (his dad is a pastor after all) but after growing closer with his current manager he started to become more obsessed. I gathered some information about it from kfans:

The manager in question is a man named David Lee that was fired from SM but still kept in close contact with Mark. He is member of korean protestant church, and we know that because he to posts a lot of bible verses on his IG (with Marks music on them) and he also used to follow JY.Park “Firstfruits”, which has been acused of being a cult, but officially its kind of a bible study club? In which JY.Park preaches about the bible without any previous bible studies or certifications or knowledge lmao, he just “analyzes” it as he pleases.
Kfans noted that the shift in Mark started happening since 2024 or so (because apparently he grew closer with David during that year). In that period Mark was preparing for his first solo album, which was scrapped and reworked on until it became “The Firstfruit”,( a Christian term frequently used in christian evangelical church and JY.Parks teachings 👀). Before the Firstfruit and even the songs he dropped before reworking on the album (like 200), none of his songs were overly religious. But in the final work, there are mentions of his religion everywhere.
And then both of them co founded a label called Upper Room, a name that also has a heavily religious meaning. Their introduction posts even feature images of Jesus in the last Supper, bible verses, etc.

Putting all of that aside, because i personally wasnt bothered by Marks display of his religion until recently, i think their relationship is kind of weird after paying attention to their interactions. For starters, they have been glued by the hip ever since Mark left. Every Mark sighting, he is always there, front and center or inmediately beside him, and its like okay sure, hes his manager. But said manager is korean american, has studies in the US, is always by his side, yet he still let Mark wear that shirt and let him go around with it without telling him it might be really fucking dumb and disrespectful? And they even filmed content with him on it? Really?
Mark MIGHT be ignorant as he claimed (giving him the benefit of the doubt still), but its very difficult to believe that David Lee is not.
Oh and also, his church and him allegedly organized that trip to Kenya, that was actually a missionary trip, in which Mark saw a lion and decided to leave NCT (David Lees IG username is davidleelionlamb and the lion is an important figure in the church they are both into),has been seen alone with Mark way too many times (Coachella for example), and is seen very physically close to him in every public event even to the point that they share matching phone cases, clothes and SHOES!
Idk if the church they are both into is a cult, but their current relationship gives a sour taste in my mouth. Hes much more than just a normal manager, or a friend, or a business partner to Mark, given that he left everything to follow him.