# his invisible qualities, namely his eternal power and divine nature, have been clearly seen 

"people have clearly understood God's invisible qualities, namely his eternal power and divine nature"

# namely his eternal power and divine nature

"that is, that his power is eternal and he is truly God"

# having been discerned in the things that have been made

"because people have understood those qualities by seeing the things that God has made" 

# they are without excuse

"people do not have an excuse" or "these people can never say that they did not know"