Why is it that the Christians who claim to believe in “once saved always saved” are the ones who respond to the altar call, pray the sinner’s prayer, and are baptized multiple times?
Because to validate their Christianity, they look to their own progress in the Christian life, rather than to Christ’s perfect life given for them. To put it another way, they let their sanctification validate their justification, rather than letting their justification validate their sanctification. One of my favorite quotes by Martin Luther talks about what happens when we try to still our consciences by looking at our own contrition, versus casting our sins on Christ:
You cast your sins from yourself and onto Christ when you firmly believe that his wounds and sufferings are your sins, to be borne and paid for by him, as we read in Isaiah 53:6, “The Lord has laid on him the iniquity of us all.” St. Peter says, “in his body has he borne our sins on the wood of the cross” (1 Pet. 2:24). St. Paul says, “God has made him a sinner for us, so that through him we would be made just” (2 Cor. 5:21). You must stake everything on these and similar verses. The more your conscience torments you, the more tenaciously must you cling to them. If you do not do that, but presume to still your conscience with your contrition and penance, you will never obtain peace of mind, but will have to despair in the end. If we allow sin to remain in our conscience and try to deal with it there, or if we look at sin in our heart, it will be much too strong for us and will live on forever. But if we behold it resting on Christ and [see it] overcome by his resurrection, and then boldly believe this even it is dead and nullified. Sin cannot remain on Christ, since it is swallowed up by his resurrection.