Our only ultimate authority is the Holy Spirit speaking in Scripture. Once you accept the Bible's authority, and start to study it, one of the first things you notice is that it is all about Jesus Christ. After his resurrection Jesus, walked to Emmaus with two of his disciples. Salvation in Jesus Christ is the message of the whole Bible, from Genesis to Revelation. When we accept Scripture alone, the Scripture itself points us to Christ alone.
Grace is unmerited favor, undeserved blessing.
God does not owe us anything except wrath. Nevertheless, he has saved us by his grace. Jesus Christ is God's gracious gift to sinners. The reason that salvation is by grace alone is because it is offered in Christ alone. But salvation is God's free gift.