The early Christians did not have the New Testament in writing, yet they were able to preach the gospel from the scriptures before the New Testament was written. For example, Peter preached from the scriptures on the Day of Pentecost (Acts 2).
The gospel is in the Old Testament scriptures, not only in the New. The outline below is an example of the kind of lessons the first Christian preachers might have given. It is a gospel sermon, yet it is a sermon from the Old Testament.
Of course we cannot do without the New Testament. But we cannot do without the Old Testament either. It shows us that the gospel was always in God’s mind and purpose.
To keep his laws (Leviticus 18:4-5).
He will look upon you as just. He will count you among the righteous (Ezekiel 18:9).
Yes. It is those who criticise the judgment of God who are unfair (Ezekiel 33:17-20).
It is either life or death depending on whether you obey his law or disobey (Ezekiel 33:18-19).
Eternal life and eternal condemnation (Daniel 12:2-3).
One must also convert many others to righteousness. To be regarded as a just person, one must lead other sinners to justification (Daniel 12:3).
It sounds simple so far, but there is a very grave difficulty. Nobody does obey God’s law properly. There is not a righteous man (male or female) on earth who does not sin. Thus, death comes to all men and no one is just (Isaiah 64:4-9 Ecclesiastes 7:20).
The one and only exception to the statement that all have sinned. This one truly just man was despised and sent to his death by his fellow men, as a lamb led to the slaughter. But he offered his perfect life as a sin offering for them all, and God accepted that through this one man who died, all men can be justified and live.
God saw that this was just: that this one perfect man should make atonement and intercession for all sinful men who come to God through him (Isaiah 53:3-12).
The same way anyone else down through the ages has ever been justified: we must have faith in this one perfect man, and follow him (Habakkuk 2:4, cf Genesis 15:6). Do you have such faith?