If you read in detail the history of Arab victories against Persia and Rome , you were amazed that they dont even have proper war uniform and equipments . They always won wars with divine support .
With poor leadership they lost a few battles as well despite having the faith, however the first Caliph quickly dismissed such commanders and assigned overall command to Khalid based on his military prowess and the rest is history.
Also, the Arabs were not rag tag warriors who had no skill.
They had the finest cavalry of the time, the Arab light horse was devastating in its stamina and speed.
They had a warrior culture in which every youngster had to excel in martial skills.
They kept their weapons sharp and in excellent condition as it was a source of pride for them, similar pride and love was shown for their horses.
Their poetry was mostly about war and its glory so it was indeed a warrior culture in which individually they were great fighters.
The only drawback and major one at that was the tribal nature and prevalent non coherent dynamics which ensured that they lacked the will and discipline to effectively create a large force.
This was the reason the Arabs were used as vassals for Roman and Persian empires and never thought about unifying and taking on the Romans and Persians who exploited their differences and inherent weaknesses in order to benefit their own empires.
However the advent of Islam changed this and when there was discipline and equality in the ranks. The Arabs transformed into a highly effective and mobile Army which easily outmaneuvered and outlasted the finest armies that Rome and Persia could throw at it.
There is no doubt in the minds of the historians as to the pivotal role of Khalid bin Waleed in making a world beating Army out of the Muslim Arabs.