Rom 8:16 says ” The Spirit Himself witnesses with our spirit that we are children of God.
It’s important to see and understand what our human spirit really is. One factor of it is that it’s the deepest part of our being that longs for God. This verse shows that the Spirit witnesses with our spirit which shows that these two spirits are actually one. The witnessing is that we are children of God and so we see that we can be one spirit with our Lord.