Luke begins this portion with the arrival of an angel of the Lord. The Lord (“Kurios”) has angels, and he sends such a messenger from heaven. The angel is probably not visible, but his voice can be heard, coming from outside Philip (or through a dream?).
Later (in Acts 8:29 and Acts 8:39) Luke mentions the Spirit, who probably gives Philip godly directions with a voice from within. After all, Philip was a man full of the Spirit (see Acts 6:3). We should not differentiate greatly between the angel and the Spirit. It is clear that Christ himself guides Philip here and that he is in charge. The Spirit guides the way, as Paul also experienced on his journey to Troas (Acts 16:6–7) and on his way to Jerusalem (Acts 20:22–23).
26 Now an angel of the Lord said to Philip, “Rise and go toward the south to the road that goes down from Jerusalem to Gaza.” This is a desert place.