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I wasn't sure whether to put this as a reply to OP directly or someone like you, but I'll try here since you seem somewhat knowledgeable about these things.
I'm not at all an expert in these things, but my understanding was that natural biogenesis from soup-of-weird-chemicals to moderately complex single-cell life forms was pretty straightforward and plausible to happen naturally. I understand this was believed to have happened within a few million years of it being physically possible, i.e. soon after the Earth formed and cooled down enough to have liquid water. The things that was more of a head-scratcher in the how in the world did this happen without divine intervention was the jump to multi-cellular life.
How does a cell that evolved to be all about itself and it's direct descendants ever decide to team up with several other cells, which all abandon their individuality and dedicate themselves to the survival of a higher-order organism? Now that seems more like a touch of a higher power. While single-celled life originated (spontaneously?) fast, the first multi-celluar organisms took billions of years to appear AIUI, and it's off to the races after that.
Apart from what other users have brought up, there's also the fact that experiments in multicellularity appear very early on in the fossil record. Our oldest evidence for it consists of macrofossils that were discovered in the Franceville basin in current-day Gabon, in what would have been a shallow oxygenated delta at the time, and which have been dubbed the "Francevillian biota" or "Gabonionta". They are dated to 2.1 Ga, in the early Paleoproterozoic.
The emergence of this biota follows the Great Oxidation Event approx 2.4-2.1 Ga, an event where cyanobacteria caused a mass extinction by producing oxygen, something which is toxic to many anaerobes. The interaction of free oxygen with cellular components produces an oxygen radical called a "superoxide anion" which is capable of triggering a chain of destructive reactions in the cell. Aerobes are only capable of withstanding this because they possess enzymes called superoxide dismutase which essentially "neutralise" the superoxide anion (and if exposed to too much oxygen can still experience hyperoxia).
Before then, Earth had a reducing atmosphere practically free of oxygen, and the GOE changed the environment into an oxidising atmosphere, with oxygen levels being as high as 10% of their present atmospheric level by the end of the GOE. And it also seems that oxygenation is a factor which is a prerequisite for the development of large multicellular organisms. Only aerobic respiration can produce enough energy for a complex metabolism, and although there are some exceptions, few multicellular life forms are anaerobic.
The Francevillian biota are surprisingly complex considering how early they appear. There are a number of forms the fossils take. Some look like elongated pearl-strings that end in a "flower". Others look like really bulbous nipples. They exhibit patterns of growth determined from the fossil morphologies that are suggestive of intercellular signalling and thus of mutually synchronised responses that are the hallmarks of multicellular organisation, and there's also evidence that they were capable of moving around in search of food resources - there are string-like tracks at the site which might represent mucus trails.
A particularly striking feature of the Francevillian biota is that they are isolated in time. No structures similar to them are known from earlier times and the biota are conspicuously absent from the overlying layer of black shale. It is notable that their disappearance also seems to roughly correlate with an occurrence called the Shunga event. What caused it hasn't been conclusively pinned down, but it involves the creation of one of the oldest known petroleum deposits on Earth, indicating the demise of a massive primitive biomass. The Shunga reserves in the Lake Onega region of Russia alone preserve up to 25 × 10^11 tonnes of organic carbon, and deposits of about the same age and having similar carbon isotope chemistry have been found elsewhere in northwest Russia, as well as North America, Greenland and West Africa, indicating that this was a global event. The organic blooms associated with the Great Oxidation Event abruptly cease, and oxygen levels drop back down to pre-GOE levels.
In short, these fossils seem to represent a first experiment in megascopic multicellularity that arose during a period of oxygenation and subsequently died off when the environment shifted against them. This seems to indicate that multicellularity can start developing relatively quickly, and part of the reason why there was a delay is because the first experiments in multicellularity were abruptly stopped in their tracks.
Which raises the question as to what would've happened had the extinction not occurred. This was a very crucial point in the evolution of life and small changes in the initial state of a system can lead to huge downstream ramifications, so how different would life be today if they had been able to develop?
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Unfortunately I don't think theism is required to explain that. The evolution of cooperation, predation, parasitism, communication, etc ("social behavior") is expected in any sufficiently complex resource-contrained environment, just as a result of game theory combined with selection. Once cells land on strategies of cooperation where they are sacrificing their own reproduction to provide resources for their siblings (in the style of the selfish gene), it isn't a big jump to multicellular organisms. According to Wikipedia, multicellularity has evolved independently at least 31 times, and complex multicellularity at least 6 times (https://en.wikipedia.org/wiki/Multicellular_organism). Since it has happened so many times, two things must be true: (1) it wasn't spectacularly improbable and (2) the genetic lines of each of those mutations has remained competitive enough to survive to this day.
I'll admit that the evolution of sexual reproduction has me stumped, though. I'm sure someone has written papers on it, and there's probably a Wiki on it, but I kind of want to puzzle over it first.
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I think your understanding is incorrect. Evolving multicellular organisms once you already have unicellular organisms is probably much easier than abiogenesis; it has in fact happened multiple times independently. Multicellularity has evolved independently at least 25 times in eukaryotes, and also in some prokaryotes, like cyanobacteria, myxobacteria, actinomycetes, Magnetoglobus multicellularis or Methanosarcina.
The same reason multicellular organisms evolve social behaviors - your relatives share a high proportion of your genes, so it is adaptive for genes to code for traits that improve your relatives' survival and propagation.
Indeed, most unicellular beings reproduce asexually, so they share 100% of genes with their kin, barring new mutations. Most instances of primitive multicellularity derive from cells dividing but remaining physically connected, so all cells in the colony are genetically identical.
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