To speak in tongues as a manifestation of the Holy Spirit, I believe you have to be saved. However I think there are other spirits out there who can imitate, and the spirits have to be tested. I heard other religions speak in tongues, but haven't looked into it.
I struggled with once saved always saved, because part of me wanted to look at the so called Christian commiting adultery, and say, well they can't be forgiven that.........but I have come to the conclusion Jesus blood is always enough. It's not a gospel of works, so whether we read the Bible 3 times a day, or once a month were still saved. And it's not like if we don't go to church for 20 weeks were ok, but one more week and were lost.........we WILL stunt our Christian walk though, by not seeking after Him.
This is not an excuse to sin and do what we like. We still need to put effort into our walk, for our sake and to His glory. God has so graciously forgiven us 'Love so amazing, so divine, Demands my soul, my life, my all.'
And we still have to account before Christ for our actions on earth, so we shouldn't desire it to be a case of kick back and hide our light.
Having prayed a false sinners prayer many years ago, I know the importance of working out my salvation with fear and trembling. And not every person who calls themselves a Christian is saved, but....... My sheep listen to my voice; I know them, and they follow me. I give them eternal life, and they shall never perish; no one will snatch them out of my hand.