Whom has God sent forth into the believer's heart — the Son or the Spirit?

God has sent forth the Spirit, specifically the Holy Spirit of promise, the Spirit of His Son, into the believer's heart. This is not about the Son being sent at the cross; it is the Spirit who indwells every believer, enabling them to cry, Abba, Father.

Les explains that after believing faith, God Himself precipitates the action. He clarifies that it is not the Son being sent now, for that concerns the cross; rather, God has sent forth the Spirit—the Holy Spirit of promise. This Spirit of His Son is sent into believers' hearts, not into a denomination or church building. Les emphasizes that you do not receive the Spirit by walking through church doors on Sunday morning. The Spirit indwells every believer, crying through the Holy Spirit's unction, 'Abba, Father.'

Galatians 4:6

And because ye are sons, God hath sent forth the Spirit of his Son into your hearts, crying, Abba, Father.

Where has God sent the Spirit of His Son—into the church building or into hearts?

God has sent the Spirit of His Son into your hearts, not into a denomination or church building. The Spirit is in the heart of every believer, not picked up by walking through a door on Sunday morning. This indwelling Spirit cries, "Abba, Father."