Truth is, both sides are clearly in the bible. If you'd be honest to look into it, casting aside any presumptions, you'd easily see that there is indeed eternal life in Christ but continuing to live in sin can put you in a dangerous spiritual condition, even lose your ability to repent.
Eternal Life in Christ
"That whoever believes in (Jesus Christ) should not perish but have eternal life." It's clear, isn't it? And when we say "eternal," it means without end. Eternal cannot be cut short. If it can be, then it's no longer eternal. It's conditional. Then John 3.16 should read: "Whoever believes in him should not perish but have conditional life." Meaning, as long as you're living within the terms of the agreement you have eternal life.
But eternal is eternal. God is eternal, and it means he has no end. He goes on. And the life we have in Jesus has the same quality--eternal. It begins now and goes on and on, without end. God's Flesh believes that if you're genuinely born again--you really surrendered your life to Christ and received him as Savior and Lord--then you are saved forever. And once genuinely saved, you're always saved. What if you sin? Then you simply genuinely repent of it and ask God's forgiveness.
There are those who believe in so-called extreme grace--saying they can live in sin after getting saved, anyway, God loves and understands them and their special relationship with him exempts them from the dire consequences of sin. It also says believers need not repent from sin anymore. God's Flesh does not believe these silly trash. We believe in eternal security and living the holy Jesus LIFE daily.
Lose One's Salvation for Eternity
You have read Hebrews 10, of course. Here's how it goes: