Can you live a Sunday-only Christianity and still spend eternity with God?
No, a Sunday-only Christianity cannot secure eternity with God. The proof is whether the Holy Spirit dwells within you. If any man does not have the Spirit of Christ, he is none of His. Dressing up on Sunday morning for an hour and hoping to spend eternity in the Creator's presence will not happen.
Les teaches that being in the Spirit means you are a whole new person, born into the very family of God. The criteria and proof is whether the Holy Spirit dwells within you. If He does not, you are on thin ice; if He does, you are solid. Paul writes in the context that if any man have not the Spirit of Christ, he is none of His. Les says this is scary, and sometimes people have to be scared a little bit and shook up. They must realize you cannot just take these things in one ear and out the other, dress it up a little on Sunday morning for an hour, and then hope to spend eternity in the presence of your Creator. It is just not going to happen. Les notes Paul makes several warnings about people living an illusion. He directs attention to Galatians chapter 5, saying people have to be faced with biblical truths because one day they will be. Paul says that in the day when Jesus Christ will judge all men according to His Gospel. Everyone will stand as lost or saved by what they have done with Paul's Gospel: that Christ died, was buried, and rose again.