Okay, let's have some thoughts on this topic. I confess that I have never understood the drive to say: "I KNOW I will go to heaven." The reason for this is quite simple: I live with the very real possibility that I may reject the grace of God. I have no question that "God has given us eternal life and this life is in His Son." I have no question that God's will for me is to remain in saving faith in my Lord until the end. He wills that for all people! But I also know that God will never coerce me in this regard. Salvation remains gift; and gifts are always rejectable.
So it seems to me that when our Lord warns: "He who endures to the end will be saved," He meant it!
St. Paul also seems to speak this way quite clearly in Philippians 3:12,13; 1 Cor 9:27.
There IS certain salvation in Christ; there is NO certainty that I will be in Christ when I die. Therefore this calls for constant and unending vigilance and attendance upon the promises of God and the Means of Grace.
Gerhard speaks of this in his meditation on carnal security where he stresses that such absolute security is not possible in any place: an angel fell in heaven; Adam fell in paradise; Judas fell under the tutelage of Christ Himself.
Note that this does not drive one to despair, but to attend to the promises of God in the preaching of the Gospel, the reception of the Sacrament, the promises of Baptism and Absolution. Because there is no "once saved, always saved" business; because our Lord could reference "those who believe for a while and then fall away" we must guard against all forms of carnal security until our dying breath.
Okay, you all. Ducking!