The subject of assurance is one of the most controversial of all doctrines. To complicate this even the definition of what the subject of "assurance" is can be mistaken.
The Oxford English Dictionary defines it as
1 a positive declaration intended to give confidence; a promise
(therefore God is the person declaring or speaking an assurance)
but a more common rendering among evangelical Christians might be:
"the inner witness of the Holy Spirit that allows the justified disciple to know that he or she is saved"
or in a different manner
However the obvious question is, "assured you are in a state of salvation now? OR does it go further and assure you that, come what may, happen as it might, you will be eventually in a state of salvation when you die, or stand before God"
Make no mistake. you must be careful understanding this issue. as it can change the good news message. There are several different beliefs about it, and the people who are in each separate group (or doctrine) are so passionate about it they will try to force their belief upon you by questioning if you are even born again, or saved, if you are not the same as them. This in itself show that doctrines about assurance can lead to false gospels, as people in these various groups declare themselves that they have the only gospel, trying to force you into making a commitment to what they believe. In short I believe that attatching an incorrect message about assurance onto the actual definition of the gospel (and thus adding to to) can leave you with a very heretical gospel. An example of this is by preaching the true gospel of believing in (trusting in and relying upon) Jesus Christ to save you, and then by saying "by save we mean you cannot be lost no matter what sin you commit, even if you never repent long term, and all the time the Holy Spirit will be assuring you of this." Adding this to the Gospel is not only heretical, it is an insult to the purity of the Holy Spirit, and is clearly false.
Lets look at the alternatives: