The Lord is merciful and gracious,
slow to anger and abounding in steadfast love.
In Jesus I know the mercy and grace of God... a mercy and grace God has always displayed as part of His many attributes. It would be heresy to think that God only became gracious in the gospels with the sending of His Son. God also did not become more merciful or gracious by sending Jesus. Anything that God is, He has always been, and that in full perfection. That is why David could praise God's mercy and grace while living under the Law in the writing of this psalm.
Thank God that He is slow to anger. Thank Him that His steadfast love abounds. Praise Him that His anger does not burn forever (Psalm 103:9). Be glad and rejoice that He does not treat us as our sins deserve (Psalm 103:10). Be amazed that His love is so great as to extend as far as the heavens reach beyond the earth (Psalm 103:11).
Yes, I know all these attributes of God through salvation in Jesus. And I am thankful. Yet God has always been faithful to His people., abounding in mercy and grace. God has always been forgiving, displaying to us daily His incomparable, steadfast love.