More often than not, we've been hearing a lot of preachers being addressed as "men of God" in countless Christian denominations world over. Rather than argue over the eligibility of men who bear these titles, I'd take the position that some are truly deserving of them, while some are not in any way close.
In past biblical times, certain men had proved beyond reasonable doubts that they are true servants and leaders ordained by Christ. Their standings with God was neatly matched with the characters they portrayed, and the works which they were known for.
Take Joseph as a case study.
He was castigated and sold out by his brothers out of jealousy, but when the time of his shine came, he shone beyond his immediate generation as the Prime Minister of Egypt. This was a man whom God had set aside for greatness, and thankfully, he never compromised on his stand when faced with the temptation of sleeping with his master's wife.
What then made Joseph a significant man in his generation and beyond?
Joseph was a man with the Saviour's pardon. This means that Joseph had a close relationship by God, which was facilitated by his Christian experience of salvation. The implication of this is pardon by God, and a call to divine service.
In addition to that, he was a man of strong persuasion, sound principles, selfless perspectives, scriptural perception and strategic programmes. These are the qualities expected of men who are called by God unto service to humanity.