Anytime somebody talks about “union with Christ” we need to ask them to define it. They should not be allowed to list various aspects of “union” but then only use one of those aspects as their working defintion. If they say that “union is by faith”, then they are identifying “union” with God’s effectual call that results in faith in the gospel.
The effectual call only takes place by means of Christ’s justification. There are not two “unions” here, one which is by faith, and another which is legal. The effectual call which results in faith is the gospel is a result of legal union with Christ in God’s imputation of righteousness.
Present union with Christ is legal. The already justified Christ gives the Spirit. It is not the Spirit who gives Christ. It is not the Spirit who makes Christ’s work effective. It is not the Spirit who makes Christ present.
We do not contemplate God’s gift of faith as getting us close to Christ so that he may justify us. Only the already elect in Christ for whom Christ died are baptised (without water) by God the Father into Christ’s death and only then do they begin to have faith in and trust Christ.
It is not effectual calling that causes righteousness to be credited to us. It is union with Christ by means of righteousness imputed and forgiveness declared that creates the effectual call to fellowship and communion.
I Corinthians 1:9–“God is faithful, by whom you were called into the fellowship of His Son, Jesus Christ our Lord.”
I Corinthians 1:30–“God is the source of your life in Christ Jesus, whom God made our wisdom and our righteousness and sanctification and redemption.”
Galatians 3:17–” in Christ Jesus the blessing of Abraham comes to the Gentiles, so that we receive the promised Spirit through faith.”
Romans 8:10–“the Spirit is life because of righteousness”
The Spirit does not give us life so that we can get righteousness. We trust Him who justifies the ungodly. Romans 4:5.