When Jesus said "Father, forgive them; for they know not what they do" he wasn't just talking about the act of crucifying him - he was talking about the hate they had in their hearts for him, and the fact that they truly did not know what they were doing because the Father had blinded them, "For blindness in part is happened to Israel until the fulness of the gentiles be come in". They could not see who Jesus was because the Father willed it because He chose to give them the honor of crucifying Christ. 

Since then, the Holy Spirit has been poured out on all flesh - consequently,  all flesh has witnessed the truth for all flesh has been touched by his Spirit,  who leads us and guides us into all Truth - so therefore all flesh is without excuse. So Jesus will not say at the last Judgment "Father forgive them for they know not", no, at that time he will say "forgive them not for they know what they've done". It's a heart issue. All hearts have been knocked on, all have been invited into his kingdom,  all have been touched by his Spirit at some point, all are without excuse.  All have the choice of welcoming or rejecting him. Whether or not they've heard the words of the Gospel- they've met the author of the words "because that which may be known of God is manifest in them; for God has showed it to them, For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse ". 

Those who respond to an altar call have already given their hearts to him - they just haven't known how to put it into words. They've welcomed the messenger before they've heard the message. Yes the Gospel must be preached but his words as seeds will only bear fruit in hearts composed of the right soil - hearts that have already been plowed by his Spirit- prepared and made ready to receive his word. Hearts that have his Word abiding in them are the ones that will live forever  in his presence  - for did he not say "so shall My Word be that goeth forth out of my mouth, it shall not return unto me void, but it shall accomplish that which I please, and it shall prosper where I sent it". When he says "my word shall not return unto me void", he is speaking of all the hearts he will gather to himself that gladly received his Word. His Word will reap a great harvest of hearts - for every heart in which his Word is sown that does not reject his word will return with his Word to him just as Jesus, the living Word made flesh, returned to him. His Word will accomplish  that which he pleases and it pleases him to save souls, not condemn them. His word brings eternal prosperity to every heart he has sent it to. Minds alone cannot contain his words because he speaks them from his heart, besides which, our minds are mortal - limited in their capacity to understand- but our hearts, filled with his Spirit, may receive his Word without limit for in his Spirit there are no limits, hindrances or boundaries. Even time and space and death itself have no hold on him, so we see that our hearts can comprehend what our minds cannot. With our minds we may only know in part till that which is perfect is come. Our hearts however, may know him in full, for to know him is to know all of him - for he does not withhold anything from us who love him. Most importantly,  he does not withhold himself. He is our inherited and we are his inheritance. 

"Teach me Thy way, O Lord;

 I will walk in Thy Truth;

 Unite my heart to fear Thy Name.

I will praise Thee, O Lord my God,

with all my heart: and i will

glorify Thy Name forevermore"