What is forbidden in the second commandment?
Firstly, the second commandment does not address worshiping other so called gods, for the worship of other so called gods was forbidden in the first commandment. The second commandment forbids the worshiping of images (man made or otherwise) as God.
In John 4:24, Jesus said to the Samaritan woman by the well:
“God is spirit, and those who worship Him must worship in spirit and truth.” (NASB)
Idolatry is creating an image that you believe to be God and bowing down to worship that image as God. When God revealed Himself to Israel, they heard a voice but they did not see a form; therefore, they were not to create an image of anything as being God, to bow down to worship that image as God, nor worship any created thing in the universe as God. (cf. Deut. 4:15-19)
Secondly, the second commandment forbids worshiping God in anyway not appointed in His word. This has happened many times throughout church history. The ascetics abased themselves thinking that this would make them closer in relationship to God, or grant them some sort of reward from God. Do not let anyone defraud you in this way. (cf. Col. 2:18)
In Galatians 5:1, the apostle Paul wrote:
“It was for freedom that Christ set us free; therefore keep standing firm and do not be subject again to a yoke of slavery.” (NASB)
In Conclusion, the second commandment forbids the worshipping of God by images, or any other way not appointed in his word.
Your Servant for Jesus’ Sake