Charlie Mulgrew also gets on the scoresheet to help Neil Lennon’s side past their First Division opponents to claim a place in the last eight of the Scottish Cup
After crashing out of the League Cup at the semi-final stage to St Mirren last Sunday, Neil Lennon’s side remain on course for a domestic double.
Raith Rovers boss Grant Murray stuck with the same side that fell to defeat away to First Division league leaders Morton, with former Celtic youngster Greig Spence having settle for a place on the bench.
The visitors made three changes from the side that thumped Kilmarnock in midweek, with Georgios Samaras and Emilio Izaguirre injured and Scott Brown rested, with Joe Ledley, Beram Kayal and Tony Watt all coming into the starting line-up.
Celtic started the first half looking to get a foothold over the cup tie and they were adamant that they should have been awarded a penalty, when the ball struck Eddie Malone’s arm as it was making its way towards Watt.
The first real chance came the way of Victor Wanyama, as the Kenyan midfielder received the ball on the edge of the Raith Rovers area, but his strike was clutched to his chest by home stopper David McGurn.
For the reminder of the first half Celtic were in total control, but the Starks Park pitch, the Kirkcaldy wind and the stubborn Raith Rovers defence all stopped the visitors from taking the lead in the Scottish Cup fifth round tie.
There were calls for a penalty from the visiting support as Simon Mensing looked to have brought down Commons inside the box, but referee Steven MacLean waved away the protests.
Celtic should have taken the lead as Watt got onto the end of a superb cross from the right by Adam Matthews, but the young striker could only direct his effort wide of McGurn’s left hand post.
Polish keeper Lukasz Zaluska was then called into action to prevent a long range free kick by Brian Graham from hitting the back of the net, palming the effort wide of goal.
Referee MacLean finally awarded a penalty to the visitors for yet another incident inside the box involving both Mensing and Commons, allowing the Scotland international to step-up from the spot and fire Celtic into the lead.
The SPL leaders should have sealed the victory when Gary Hooper jinked through the middle of the Raith Rovers defence, before unleashing a shot which McGurn could only parry into the path of substitute Brown, but his effort went the wrong side of the post.
With the second half running down Celtic secured the victory as substitute Forrest got in behind the home defence, round McGurn and stuck the ball.
The icing on the cake for Celtic came with four minutes left as Ledley played the ball back to Mulgrew on the edge of the area and the Scottish international curled the ball beyond the outstretched arms of McGurn to seal the passage into the quarter-finals.