what we have is His everlasting and never-changing PROMISE.

when we have doubts on anything, such as,
can we guarantee 信到底 or 忍耐到底?
we have to go back to His PROMISE: The Holy Spirit living me
PROMISED to help me 信到底 or 忍耐到底, thus, I believe I can 信到底 or 忍耐到底!
not because I promise to be strong to guarantee me 信到底 or 忍耐到底,
but because Jesus Christ promised to make me 信到底 or 忍耐到底, and how?
I don't know, it doesn't matter as long as I believe
His grace is always sufficient!

you will ask why Jesus Christ didn't promise to make those ex-christians 信到底 or 忍耐到底, I don't know either,there is an explanation in history (predestination), which is like of ZhuGe Liang after the event, yes, this is what human being can do the best.

SEE, we are at the corner now, in fact, all 'debatable issues' in those
sound doctrines would automatically lead people to the corner, where nowhere to go
but to choose by faith:

God decide ...
or
I decide...

God decide 您 能信到底 or 能忍耐到底
or
您 decide 您 能信到底 or 能忍耐到底

I believe God decide 您 能信到底 or 能忍耐到底, so 必然得救 because of His Promise
您 believe 您 decide 您 能信到底 or 能忍耐到底, so 不确定 because you change

KEY:

God have the sovereignty
or
you have the sovereignty