With both of them dead Empire is left with only Licinius and Maximinus Daia as Emperors... And since these 2 were strongly pagans they would have reestablished harsher Christian persecutions...
I agree that Licinius and Maximinus Daia are unlikely to embrace Christianity. But I'm not sure about renewed or increased persecutions. Licinius did play around with the idea OTL during his co-emperorship with Constantine, but I suspect that much of that was done to present himself in opposition to Constantine. With Constantine gone before he ever really had a chance to make his favor for Christianity known, there may not be much impetus to persecute.
I certainly don't think Christianization is inevitable. Far from it, actually. I ascribe to the school of thought that has Constantine's conversion as much of a personal act as a political one, and I tend to think that it was Constantine (and his heirs) successes as emperor that made Christianity the dominant religion of the empire, and not vice versa. I'm also not convinced that classical paganism is doomed to die out. Cults like those dedicated to Mithras or Sol Invictus were not new to Rome, and had quite easily coexisted with the Olympic dieties for centuries. Christianity was fairly unique in its monotheism, after all. I suspect that the division of personal and civic faith would continue apace: soldiers might follow Mithras on their own, but Jupiter et al. would continue to hold an important place in the public sphere.
That said, I also don't think Christianity was a major factor in the decline of the empire. I find it a downright ludicrous to think that the number of severity of civil wars increased following Constantine's conversion: the third-century crisis tops anything post-Constantine, and none of the major players there were Christian. The late Roman world and the Byzantine world had plenty of military coups and usurpers, and very few of these conflicts were played out as holy war. Even in the handful of exceptions (like the iconoclasm controversy), religious justifications tended to be little more than overlays in place of more traditional social and political rifts. The army supported iconoclasm not out of deep-seated ideological belief, but because iconoclastic emperors were successful on the battlefield, and the social divisions between Rome and Constantinople would have existed regardless of whether or not they found a convenient exemplar in the veneration of idols.
I think the ultimate fall of Rome (in the west, at least) had much, much more to do with long-term demographic changes, like the gradual decline in population of the western half of the empire, the increased number and sophistication of Germanic groups outside of the Rhine and Danube borders, the emergence of more deadly diseases, and a resurgent empire in Persia, than any purely religious factor. If anything, I think Constantine's reign bought the empire more time than it would have had otherwise. Diocletian's reforms were a start, but before Constantine the empire was already beginning to slide back into the pattern of civil war and external invasion that had characterized the third century. True, not all, or even most, of this is due to Christianity, but if you get rid of Constantine, it's hard to see a long future for Rome.
The one thing Christianity did provide was a mechanism for some aspects of Roman culture to persist following the political collapse of the empire. Without it, Europe is likely to drift apart much more noticably than it did OTL. Barbarian successor states probably still seek to emulate as much of Roman culture as they can, but over the centuries they evolve down much different paths without the long shadow of either Rome or Constantinople to unite them in spirit, if not in fact.