It depends on what you mean by "believe".
I agree that salvation is simple. But I disagree that a moment of mental assent, for example, is all that is necessary. (I'm not saying that you're saying that.)
If we believe in Christ, we believe what He says, and we will do it - to the best of our ability. It's not about perfection, it's about following Christ, doing what He says, depending on Him for our salvation and trusting in God's mercy, repenting when we fail and keep at it until we die. Living the life of a follower of Christ, in other words, cooperating with the grace of God to be transformed into the likeness of Christ.
There are many things we can do in this process, many tools we can use, many ways God can extend His grace to us. But it's not a formula or checklist. Just that God loves us and WANTS to save us, and we (hopefully) cooperate with Him and don't reject His work in us.