Freeroll MTT- Ad2d - to call or not to call?

This was the first hand dealt on a freeroll. I was on the button and limped along with the other players and saw the flop. When checked back to me I decided to raise half the pot, since I thought that any one else holding an ace would rather have bet, but other pairs might call with a T or 7(yes, it was a freeroll--many guys just go all-in with any two). I figured that if someone else was holding an A I would get raised and then would fold the hand. After my bet the other 4 guys on the hand called (how many aces are in a deck of cards? hehe). Another spade comes on the river, making a potential flush-draw to go along with the straight draws. So my next bet, if checked back, would be pot-sized to drive at least two players off the pot, or make them pay for their draw. But then MP shoves. From the way he played the hand I thought he was weak. He could have been slow playing two-pair, trips, or even AK, but I thought that from the way he had played pre-flop(call-call) and at the flop(check-call) the best hand he could have would some lower pair with a flush draw, or a flush and straight draw. There were still two players to act behind me, but I thought they would just fold, or call with a lower pair and a draw, just like mister MP here. Still, calling all-in on the first hand of the tournament would be a pretty bad play with A2, right?So I was kind of puzzled, feeling that I shouldn't call, but thinking that I should call.

PartyGaming - $0+$0|10/20 NL - Holdem - 6 players
Hand converted by PokerTracker 3

BB: 3,000.00
UTG: 3,000.00
MP: 3,000.00
CO: 3,000.00
Hero (BTN): 3,000.00
SB: 3,000.00

SB posts SB 10.00, BB posts BB 20.00

Pre Flop: (30.00) Hero has 2♦ A♦

fold, MP calls 20.00, CO calls 20.00, Hero calls 20.00, SB raises to 40.00, BB calls 20.00, MP calls 20.00, CO calls 20.00, Hero calls 20.00

Flop: (200.00, 5 players) A♠ 7♥ T♦
SB checks, BB checks, MP checks, CO checks, Hero bets 100.00, SB calls 100.00, BB calls 100.00, MP calls 100.00, fold

Turn: (600.00, 4 players) 3♠
SB checks, BB checks, MP bets 2,860.00 and is all-in,