Salvation - The rescue or redeption of something or someone from a penalty, judgement or form of danger.

Christ died for the salvation of humanity from the penalty of sin (Romans 5:8,6:23).

The problem - sin (Isaiah 59:1-2).

The penalty - death- physical death (Ezekiel 18:20) and spiritual death, which is separation from God (Romans 6:23)

The payment necessary - innocent blood must be shed to atone for sin.

A. In the Old Testament , animal sacrifices were made to cover over sins (Leviticus 17:11)
B. In the New Testament , we learn that those sacrifices were a foreshadowing of the perfect sacrifice of Christ (Hebrews 10:1-14)

Why can't a man pay for his own sins?

A. God is Holy and cannot tolerate sin in His presence (Habbakuk 1:3a)
B. Sin separates us from Him (Isaiah 59:1-2)
C. Because of the tainted sin nature which humanity carries, we can't even pay for our own sin.

The payment given - the shed blood of Christ (1 John 1:9)

The result - Eternal Life (John 3:16), no condemnation (Romans 8:1), justified (Romans 5:1), Sealed (Eph. 4:30, 2 Cor. 5:5), eternally secure (John 10:27-28).

How this payment is obtained on your behalf - Believe (to actively put your trust in) upon Jesus Christ and accept His sacrifice on your behalf for sin. (Acts 16:30-31, John 3:16)