That's all that is necessary. Anything we add to that is just a bunch of fluff. The free and full forgiveness of sins is a gift of God on account of Christ not because we repent and believe, but so that we might believe and repent. I'll say it again. The free and full forgiveness of sins is a gift of God on account of Christ not because we repent and believe, but so that we might believe and repent.
Scripture is clear.
Anything we attempt to do to gain God's favor so that we have this promise of everlasting life is not enough. It won't do it. That's why Lutherans have had so much trouble with the Roman Catholic practice of indulgences. They are understood as somehow cooperating in our forgiveness, so that we earn the forgiveness of God in some way. That's why Lutherans have had so much trouble with altar calls of some denominations.