Surge of faith effects all units in its radius. If the unit is an enemy, it takes damage. If it is an ally, it gains move and attack speed. This does not just boost your minions. If they are not in the radius they will not attack any faster.
However, because the MOVEMENT SPEED of your minions is the same as your OWN movement speed, then they will always get the speed boost, since Surge will always effect you.