Luke 1:1
Forasmuch as many have taken in hand to set forth in order a declaration of those things which are most surely believed among us, Forasmuch as many have taken in hand to set forth in order a declaration of those things which are most surely believed among us,
Luke opens by noting that many had taken in hand to undertake and arrange an orderly account of the events of Christ's life. What he describes is a declaration of those things most surely believed among believers, shared and firmly established convictions and truths held in common. His own Gospel therefore stands among those written accounts setting forth the life of Christ.
Those Gospel accounts accurately record events witnessed by the apostles. They preserve the truths Jesus spoke. They also show things to come. God Himself enabled the apostles to write this record.