Eusebius informs us in his Life of Constantine (Ch. XII) that during Constantine's battles against Licinius, he pitched a tabernacle with a Cross at a distance from his camp where he would pray in times of battle. Eusebius writes:
But while Licinius, giving himself up to these impieties, rushed blindly towards the gulf of destruction, the emperor on the other hand, when he saw that he must meet his enemies in a second battle, devoted the intervening time to his Saviour. He pitched the tabernacle of the cross outside and at a distance from his camp, and there passed his time in a pure and holy manner, offering up prayers to God; following thus the example of his ancient prophet, of whom the sacred oracles testify, that he pitched the tabernacle without the camp. He was attended only by a few, whose faith and pious devotion he highly esteemed. And this custom he continued to observe whenever he meditated an engagement with the enemy. For he was deliberate in his measures, the better to insure safety, and desired in everything to be directed by divine counsel. And making earnest supplications to God, he was always honored after a little with a manifestation of his presence. And then, as if moved by a divine impulse, he would rush from the tabernacle, and suddenly give orders to his army to move at once without delay, and on the instant to draw their swords. On this they would immediately commence the attack, fight vigorously, so as with incredible celerity to secure the victory, and raise trophies of victory over their enemies.
Eusebius further on mentions in Ch. LVI how in his battle against the Persians, Constantine brought with him bishops as well as a tent made in the form of a large church this time.
It is also worthy of record that about the time of which I am at present writing, the emperor, having heard of an insurrection of some barbarians in the East, observed that the conquest of this enemy was still in store for him, and resolved on an expedition against the Persians. Accordingly he proceeded at once to put his forces in motion, at the same time communicating his intended march to the bishops who happened to be at his court, some of whom he judged it right to take with him as companions, and as needful coadjutors in the service of God. They, on the other hand, cheerfully declared their willingness to follow in his train, disclaiming any desire to leave him, and engaging to battle with and for him by supplication to God on his behalf. Full of joy at this answer to his request, he unfolded to them his projected line of march; after which he caused a tent of great splendor, representing in shape the figure of a church, to be prepared for his own use in the approaching war. In this he intended to unite with the bishops in offering prayers to the God from whom all victory proceeds.
Socrates Scholasticus, in his Ecclesiastical History, also mentions the transportable church of Constantine the Emperor. In Ch. XVIII he writes:
So great indeed was the emperor's devotion to Christianity, that when he was about to enter on a war with Persia, he prepared a tabernacle formed of embroidered linen on the model of a church, just as Moses had done in the wilderness; and this so constructed as to be adapted to conveyance from place to place, in order that he might have a house of prayer even in the most desert regions. But the war was not at that time carried on, being prevented through dread of the emperor.
Sozomen, in Ch. VIII of his Ecclesiastical History, continues this subject by writing the following:
When he engaged in war, he caused a tent to be borne before him, constructed in the shape of a church, so that in case he or his army might be led into the desert, they might have a sacred edifice in which to praise and worship God, and participate in the mysteries. Priests and deacons followed the tent, who fulfilled the orders about these matters, according to the law of the church. From that period the Roman legions, which now were called by their number, provided each its own tent, with attendant priests and deacons.
Other sources also, basing themselves on these above, mention the transportable church of Constantine the Great, which shows his great piety as an emperor of the Romans.