Assurance is Eternal Security.

Common sense should tell you that if you didn't have the power to save yourself it would be unlikely that you would have the power to unsave yourself.

God saved you by grace through faith in the person of Jesus. To undo the work of Christ wouldn't you have to be greater than Christ?

Paul describes our security as eternal. His language is unambiguous.

"Who shall separate us from the love of Christ? Shall tribulation, or distress, or persecution, or famine, or nakedness, or peril, or sword? ...we are more than conquerors through Him who loved us. For I am persuaded that neither death nor life, nor angels nor principalities nor powers, nor things present nor things to come, nor height nor depth, nor any other created thing, shall be able to separate us from the love of God which is in Christ Jesus our Lord." (Romans 8:35-39)

How then can any believer suggest that a Christian can 'lose' his salvation?

With absolutely no insult intended, one problem is we haven't been reading carefully enough. As we read, the Holy Spirit is guiding us into all truth. Our responsibility is to properly process the information.

"Be diligent to present yourself approved to God, a worker who does not need to be ashamed, rightly dividing the word of truth." (2 Timothy 2:15)

If we fail to rightly divide the word misunderstandings are inevitable. To avoid the kind of confusion that's inferred in the question above we have to make some distinctions that you may not have noticed before. For example, the New Testament talks about a reward and a gift. Those aren't the same thing.

A reward is something that is earned. A gift is something that is given.

The reason you might not have given much thought to the difference between the reward and the gift is probably because you've been taught that Heaven and the Kingdom are the same thing.

But that can't be right, because, if that were true, it would mean that in order to get into Heaven, we would have to both receive a gift and earn a reward.

Hmm. If I'm working for Heaven what happened to God's gift of Heaven? Heaven is a gift of God's grace so we must be working for something else.

Some believers (literally: the 'overcoming partakers') will be rewarded by serving with Jesus in a specific future event that He calls the Kingdom.

Many are called, few are chosen' makes sense in a Kingdom application. If you're giving your heart to God in the midst of a world that hates Him, and therefore you, you should be rewarded. Approved workers and overcoming partakers sound like the kind of believers who would be most usefull to Jesus.

So the gift is heaven. The reward is the Kingdom. We don't get to Heaven by works because Heaven is a gift of grace. If Heaven were a reward for our works we'd have reason to boast. Paul is clear that none may boast.

"For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, not of works, lest anyone should boast." (Ephesian 2:8-9)

According to that verse Heaven can't be the reward.