John 3:16
The instant you believe, *Bingo!*, you're in.
The other part of your question is whether or not it's possible to fall from this state of grace. That question requires a boxing ring and syndicated talking heads.
So, say I accept Christ and aknowledge His work on the cross and become a devout Baptist or a Catholic or Lutheran and then one day I decide to become a member of the Church of Jesus Christ of Latter-Day Saints. I'm still saved in your opinion, right?