The Roman soldier was expected to keep one thing in view, and only one the service of his commander. He was not allowed to marry, nor could he engage in agriculture, trade or manufactures. He was a soldier, and could not be anything else.
The figure is very suggestive of the singleness of aim which characterizes the true minister of Jesus Christ. He is not allowed to engage in any employment which will, by its entanglements, interfere with his usefulness. This is what Paul, in the text, designs to intimate to Timothy.