The word gospel literally means "good news."
Specifically it is the good news that God provided the way of salvation from sin for us through the sacrificial death and resurrection of His Son, Jesus Christ, and that by believing in that good news we can have the forgiveness of sins and eternal life.
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There is no other truth in Christianity as important as the gospel.
Why? Because that is how we begin our relationship with God and it is the foundation for all that comes later:
"So then, just as you received Christ Jesus as Lord, continue to live your lives in him." (Colossians 2:6).
We received Christ as Savior by faith. And we live our lives as disciples by faith each moment of each day. We need to grow in that same faith!
"Rooted and built up in Him, strengthened in the faith...." (Colossians 2:7).
"Grow in the grace and knowledge of our Lord and Savior Jesus Christ..." (2 Peter 3:18)