Next time a Christian asks you whether you have assurance to
Nirvana... tell them this:
Assurance? Yes, Nirvana is assurable. This is Buddha's good
news. Just believe (have right view of reality) and be saved.

(but of course, don't stop at belief, go and actually see it for
yourself!!)
To stress the importance of right view... I would like to quote
what the Buddha said.
http://www.accesstoinsight.org/lib/study/stream2.html#arising
...."Form... Feeling... Perception... Fabrications...
Consciousness is inconstant, changeable, alterable.
"One who has conviction & belief that these phenomena
are this way is called a faith-follower: one who has entered the
orderliness of rightness, entered the plane of people of integrity,
transcended the plane of the run-of-the-mill. He is incapable of
doing any deed by which he might be reborn in hell, in the animal
womb, or in the realm of hungry shades. He is incapable of passing
away until he has realized the fruit of stream-entry.
"One who, after pondering with a modicum of discernment, has
accepted that these phenomena are this way is called a
Dhamma-follower: one who has entered the orderliness of rightness,
entered the plane of people of integrity, transcended the plane of
the run-of-the-mill. He is incapable of doing any deed by which he
might be reborn in hell, in the animal womb, or in the realm of
hungry shades. He is incapable of passing away until he has
realized the fruit of stream-entry.
"One who knows and sees that these phenomena are this way is
called a stream-winner, steadfast, never again destined for states
of woe, headed for self-awakening."...
On the topic of Right View, it is said that one who realizes the
nature of dharma is said to enter the stream, become a stream
enterer, destined to attain Nirvana/Arhatship in no more than 7
lifetimes (or in this lifetime if he works hard for it). He will
never again enter into the lower realms. He no longer has self-view
(he realizes Anatta), he no longer has doubts about Dharma, he no
longer pays attention to meaningless rituals.
However, much is also said about someone who has not
attained stream entry, but simply a 'faith-follower' having the
Right View of the nature of dharmas - as being Anicca, Dukkha, and
Anatta - Impermanent, Unsatisfactory, and Non-self.
And having the right view of Dependent
Origination/Emptiness.
That is, if you understand, have conviction and belief that the
nature of dharma is so... You will not die until you attain
stream entry.
Therefore, by simply having the right view you have
already gone half the way of your path. This is why Right View is
the 1st of the Noble 8 Fold Path.
By having the Right View, your stream entry, your awakening into
the nature of dharma is assured this lifetime and thereby your
Nirvana is assured to happen in no more than 7 lives.
This is the importance of having the right view.
On another note... I have seen many (really, a lot) of very
sincere practitioners who practice very hard... much harder than
me. Unfortunately, because they do not have the right view, they
get stuck at a certain stage of experience or realization.
For example, they may get stuck at the 'I AM' level of realization. Why? Because there
is no one to point them out the right view... So no matter how hard
they practice, they cannot go pass that stage... even if they
practice for decades or even after they pass away.
Whereas, for me, and many others... who are not very diligent
practitioners, but somehow due to having been instilled the right
view, certain conditions arise and the nature of Dharma is seen
very quickly. In a matter of few years, it can be done.
Never think you can skip 'right view' and just practice
and hope that one day you will simply realize things by yourself (a
very common mistake, I believe)... cuz, no matter how hard you
practice, you still probably won't realize the right view by
yourself. You need to understand and have conviction in the
Buddha's teachings.
So please... if there is any doubts or things you don't
understand about the view of Dharma, please get it sorted out,
please have the right view.
If you don't understand Anatta (no-self), or Emptiness, please
ask.
There are experienced moderators... like Thusness and Simpo who
can point them out to you..
It is really not difficult to grasp this... so make the effort,
it is definitely worthwhile.
Don't you want to be assured enlightenment in this life? Yes,
this assurance is possible. You just need to have the right view.
(Of course, right practice, i.e. direct contemplation, is also
important as a follow up to attain enlightenment, but even right
view alone ensures your enlightenment within this life)
I am especially indebted to Thusness who pointed out to me the
'right view'... otherwise I will probably get lost in certain phase
in my practice like I mentioned earlier.