Q:

The Mormon church claims to be the true church and yet there have been billions of people that have become Christian in the last 2,000 years based on what Jesus Christ did on that cross.

The Apostle Paul put it this way at 1 Corinthians 15:1-4, "Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand, vs2, BY WHICH YOU ARE SAVED if you hold fast the word which I preached to you, unless you have believed in vain, vs3, For I delivered to you as of first importance what I also received, that Christ died for our sins according to the Scriptures, vs4, and that He was buried and that He was raised on the third day according to the Scriptures."

So why do we need "Another Testament of Jesus Christ" when the first testament, i.e the Bible, (specifically the New Testament) has proven sufficient to save one's soul?

In fact, Jude 3 says, "Beloved while I was making every effort to write you about our common salvation, I felt the necessity to write to you appealing that you contend earnestly for the faith which was once for all delivered to the saints/holy ones."

Jude is urging Christians to struggle to defend the Faith. The Greek word "Epagonizesthai" comes from the fierce competition of the athletic field. Believers must fight with all their strength to preserve "the faith" which was handed down to them. "Hapax" means "once for all." The gospel is fixed, not to be revised or have somebody else with another "gospel/testament" show up.(Galatians 1:8-9).

A:

Mormonism teaches that salvation comes through Jesus Christ, like any Christian faith.

A:

What does Mormonism have to offer in regards to salvation that Jesus Christ had not already accomplished?

In some sense, nothing. The atonement of Jesus Christ is what saves us. Faith in Jesus Christ is what saves us. The scriptures, however many they may be, can only lead us to Christ but never save. An illiterate person can be saved. But in this sense you have to question what any denomination has to offer, or what the visible church had to offer that made it a good idea to establish one in the New Testament. Or even what 98% of the Bible "has to offer".

However, Jesus has also said that we need to be baptized (John 3:5). In his time, he gave his apostles authority to baptize in his name. Without this authority, a baptism is just going through the motions. We believe this authority was lost early in the church history and restored by angels who conferred this authority to Joseph Smith. So in that sense, the true church should have authority to perform ordinances that Christ described as necessary for salvation.

The Church of Jesus Christ of Latter-Day Saints claims to be a restoration of the original church Christ founded. Literally the church of Jesus Christ. Whatever it was that having a church in the first place offered, is what "Mormonism offers".