Why do we think salvation comes only through response to an altar call?

It has become popular to believe that all there is to receive Christ and become a Christian is a response to an altar call, where one professes their belief that Christ paid for their sins and is now their Savior. 

According to the Bible, however, such a profession is expected to be just the starting point. There is a "holding on", a "remaining steadfast" that is required after this point for the work of salvation to be perfected in an individual's life.

Don't believe me? Well, here are some scriptures to back them up. These scriptures are all from the Epistles of the New Testament, which were primarily addressed to believers:

NB: I utilize different bible versions for this exposition because I want to make this message as clear as possible. I tried to go for the version that had the least complicated language usage in each instance.

Colossians 1:21-23 (New International Version)

Once you were alienated from God and were enemies in your minds because of your evil behavior.
But now he has reconciled you by Christ’s physical body through death to present you holy in his sight, without blemish and free from accusation—
if you continue in your faith, established and firm, and do not move from the hope held out in the gospel. This is the gospel that you heard and that has been proclaimed to every creature under heaven, and of which I, Paul, have become a servant.

2 Peter 3:17-18 (Easy-to-Read Version)

Dear friends, you already know about this. So be careful. Don’t let these evil people lead you away by the wrong they do. Be careful that you do not fall from your strong faith. But grow in the grace and knowledge of our Lord and Savior Jesus Christ...

Hebrews 10:35-36 (The Living Bible)
Do not let this happy trust in the Lord die away, no matter what happens. Remember your reward! You need to keep on patiently doing God’s will if you want him to do for you all that he has promised.

1 John 2:24-25 (The Message)

Stay with what you heard from the beginning, the original message. Let it sink into your life. If what you heard from the beginning lives deeply in you, you will live deeply in both Son and Father. This is exactly what Christ promised: eternal life, real life!

2 John 1:8-9 (New International Version)

Watch out that you do not lose what we have worked for, but that you may be rewarded fully. 

Anyone who runs ahead and does not continue in the teaching of Christ does not have God; whoever continues in the teaching has both the Father and the Son.