Since anybody can become a Christian in an instant, nothing he does can ever sabotage irretreivably that decision, and cannot be compelled to abandon this choice - how come there are so few Christians?
One reasons is that non-Christians, including self-identified 'Christians', fail to repent their sins. The devil can usually make us sin, but cannot stop us from repenting - we must do that to ourselves. In any case, everyone sins - so that sin does not matter of itself (Christ came to save sinners).
Anyone who repents his sins is immune to Satan! But anyway people fail to repent; even though they always could and nothing could stop them.
They usually fail because they fail to identify sin as sin; and instead regard their sin as either irrelevant or as virtue. Repentance entails recognition - and it is this recognition that is typically lost.
This is value-inversion, and is dominant in the world today as never before; because top-down, official, propagandized and enforced.
Anyone determined to follow Jesus Christ to resurrected eternal life in Heaven - and put that as his first priority - cannot be prevented from attaining this by any power on earth, or Hell.
So what goes wrong so often?
Typically, that the individual Does Not Want resurrected eternal life in Heaven; but wants something else instead.
Even among self-identified Christians, it is evident that many do not want what Jesus offered - but either some modification or something else altogether.
Satan cannot prevent someone who wants resurrection from achieving it, but he may be able to persuade people not to want it - and he does.
Another reason is the so-called Christian Churches; and the idea that one can only be a Christian if some or another of the Churches gives you the green light (and the idea that if you leave, or break the rules of, that Church - then you are no longer a Christian).
Yet, as of 2020, the leadership of the major Christian Churches has joined the side of Satan - and most Church members are willing to go along with this. In the spiritual war of this mortal life, the Churches have taken the side against God.
How come? Partly by decades of cumulative corruption of the concept of 'what it is to be a Christian', such that now the primary and essential requirements include to support strategies of sin...
And partly by pretending to fight the spiritual (and now also material) war for one or another peripherally or partially Christian-compatible reason; when in truth the war is organized against Christ himself.
This being the 'Antichrist' mode of operation.
In sum; Jesus arranged things so that it is easy to become and to remain a Christian - and no earthly power can prevent this.
But earthly powers can - if you let them - convince you not to want to be a Christian, and/or can persuade you that being-a-Christian is something other than what it is.
The choice is yours.