My guess is that the people described in the sentence saw a divine omen of some kind.
The sentence starts with an absolute ablative quo viso, "that thing having been seen".
In more fluent English it is best adapted to the subject of the sentence: "Having seen that thing, they…"
So, the people had seen something.
They were ignorantes, ignoring, of the fact indicated by a short indirect question: quid esset.
That is, they ignored or did not know what it was that they saw.
In addition to ignorantes, they were committentes (two Ts in classical Latin), "committing".
They committed themselves (se) to the God (Deo).
I don't know the most idiomatic English phrasing in this situation, but they entrusted themselves in God's hands.
Putting the beginning of the sentence together, the had seen something, they didn't know what it was, and they trusted the God.
Your translation starts from this point.
It is good, but I want to clarify some points:
- They went towards "a city (which is) called Marescum".
- They made a pause and rest (mora et quies, again an absolute ablative).
- The superlative maximam does not mean that it was the biggest city; especially with quam it means that the city was very large, but no comparison is implied. One could read urbs quam maxima as "largest possible city", but "an extremely large city" sounds much more likely to me. (Literally, urbs quam maxima can be read as "a city as big as possible", but such a hyperbole is not idiomatic English. There are many choices for the exact phrasing, but I don't think a strict comparison is intended.)
- I assume it is Syriae, not Syria, since I cannot parse a nominative or an ablative here, and a genitive would make sense (and you translate it as a genitive).
For more natural English, I prefer to split the sentence in two.
Combining all this, I would translate your sentence as follows:
After seeing it but not knowing what it was, they, leaving themselves in God's hands, set out [from there] towards a city called Marescum/Marash.
After pausing there to rest for three days, they continued towards Antioch, a very large city of Syria.
I assumed that the predicate profecti sunt is implied to cover also versus Antiocham.
For fluency, I translated this (implicit) second occurrence with "they continued".
I might be able to offer a better translation for the start if I knew what they had seen.
There is a worse alternative (mis)reading that I did originally.
I include it here for completeness.
If you read commitentes as comitantes, then the people were "accompanying".
They accompanied themselves (se) to the God (Deo).
That is, they were in the company of the God.