Is it true that at some point god will not forgive your sins and how do you know
if you are one of those?
Jesus tells us in the New Testament that believing in Him and true repentance of your sins is what will allow us to be in the Kingdom of Heaven. That the rejection of Him will condemn us. For those that go their entire life not believing in Jesus, damnation is their fate;
"Whoever believes in him is not condemned, but whoever does not believe stands condemned already because he has not believed in the name of God's one and only Son" John 3:18
Is there a 'point' in your life where this damnation is pronounced on you? I believe the answer to that is if you die as a unrepentant sinner, then there is truly no hope for you. Can someone at the 'last minute' come to repentance and be saved? Yes, I believe so, if the repentance is true within their hearts. Look at the sinner on the cross, at the last minute before his death, He asked Jesus to remember him. Jesus' response was;
“And Jesus said to him, "Assuredly, I say to you, today you will be with Me in Paradise." Luke 23:43
By just saying "I believe" to 'get you in' I believe won't work. God knows what's truly in the hearts of men. He is the final judge. If you do believe in Jesus, and God's word as the truth, but continue in doing what you know is wrong, I believe this is going to be a problem for you.
Jesus also tells us that to be watchful. That the day will come when His judgment is upon us, then, I believe, it's to late;
"Watch therefore, and pray always that you may be counted worthy to escape all these things that will come to pass, and to stand before the Son of Man." Luke 21:36
With the above having been said, I do not believe that man can decide who or when one will be saved simply because man cannot tell what is within another ones heart. Only God can. The final judgment is His.
There is also a condition that can occur when a sin of a certain kind is done you will not be forgiven. This is told to us in the book of Hebrews which sates, that after coming to the knowledge of God's word, we willfully sin, this will not be forgiven;
“For if we sin willfully after we have received the knowledge of the truth, there no longer remains a sacrifice for sins, but a certain fearful expectation of judgment, and fiery indignation which will devour the adversaries." Heb. 10:26-27