The Englishman was this week named in Roy Hodgson's squad for the Three Lions' upcoming qualifiers with Moldova and Ukraine.
His place in the group for next month's double-header only opened up after West Brom stopper Ben Foster was ruled out for 12 weeks with a stress fracture to his foot.
But the 25-year-old believes he will only be able to keep hold of that slot until next year's finals in Brazil if he is playing at the highest level possible for the Hoops.
"It's obviously a World Cup year and it's crucial to be playing Champions League football, especially at a club with the calibre of Celtic," said Forster after Celtic secured their place in the group stages with the 3-2 aggregate win over Shakhter Karagandy.
"It will be a fantastic opportunity again to test ourselves against the best players in the world, which is something we all want to do.
"It's been a good week, I'll take that. It's been brilliant. It's obviously nice to be in the England set-up again.
"Every season seems to be big and I just need to keep myself going. We had a fantastic year last season. We have lost a few players since then but it has been key for us to get in the Champions League again and it keeps everyone moving forward."
Like most of his team-mates, Forster refused to speculate too strongly on who the Parkhead side might be paired with in Thursday afternoon's group stage draw. But he did admit landing an English side would special to him.
"There's quality throughout the whole draw," he said. "It might be nice to get an English team but we will just wait and see who we get."
Forster was a virtual spectator for much of a breathless encounter with the Kazaks on Wednesday night.
But he was beaten twice in the space of a couple of minutes, only for first Adam Matthews and then the crossbar to rescue him just as Shakhter looked set to burst Celtic's bubble.
Kris Commons had reduced the visitors two-goal first-leg lead to one with his fierce strike just before the break, while Georgios Samaras had the Scots level within three minutes of the restart.
However, with extra-time looming and Parkhead nerves fraying by the second, James Forrest struck a last-gasp winner to send the stadium rocking.
But Forster insists he was never worried, even when Shakhter came too close for comfort.
He said: "We were obviously attacking quite a lot, so that is going to happen. At the end of the day we kept a clean sheet and scored three. I can't really wish for better than that.
"We have a fantastic group of players. We all want to play for each other and give 100 per cent.
"We have a great team spirit - probably the best I've ever experienced. You could see that after the final whistle.
"It was a fantastic result and a lot of that is down to the coaching staff and the way they prepared us for the game. We're just delighted to be back in the Champions League."
Forster, who was a key figure in last year's run to the last 16, also hailed the raucous atmosphere created by the Celtic Park support.
He said: "Every Champions League night here is huge. The fans were brilliant and were a definite 12th man for us.
"They really spurred the lads on and the fans can definitely take some credit for the result.
"Last season is been and gone. It's all about this season now. We wanted to be back in the Champions League and that's what we have done.
"We have probably made it a wee bit hard for ourselves but that's just how we seem to do it."
- Sports & Recreation