For I [Apostle Paul] through the law [customs] am dead to the law, that I might live unto [for] God. {20} I am crucified with Christ [the Anointed One]: nevertheless [but] I live; yet not I, but Christ liveth [lives] in me: and the life which I now live in the flesh [body] I live by the faith of the Son of God [Jesus Christ], who loved me, and gave himself for me.
King James Version
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One way that we have been crucified with Christ is by relationship: we have become one with Christ, and His experiences are ours.  Our Christian life began when, in unity with Him, we died to our old life.  In our daily life, we must regularly crucify sinful desires that keep us from following Christ.  This, too, is a kind of dying with Him.