Actually, it’s not complicated, but it isn’t easy, either. The theology of salvation is not the same as the theology of redemption. A lot of people think they are one and the same thing, but they aren’t.
Christ has finished the work of redemption when he died on the cross, rose again and presented himself as the offering for our sins to the Father. A done deal, so to say. That isn’t complicated at all.
Salvation, OTOH, is not guaranteed to us. Christ’s redemptive act makes salvation possible, but we have to cooperate with God’s grace in order to obtain it. As St. Paul admonished: Phil.2:12 Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling.
Also, St. Peter warned us: 2Pt. 2:20 For if, after they have escaped the defilements of the world through the knowledge of our Lord and Savior Jesus Christ, they are again entangled in them and overpowered, the last state has become worse for them than the first.
21 For it would have been better for them never to have known the way of righteousness than after knowing it to turn back from the holy commandment delivered to them.
22 It has happened to them according to the true proverb, The dog turns back to his own vomit, and the sow is washed only to wallow in the mire.
This is only a couple of biblical admonitions to keep faithful to Christ or suffer the consequences. It’s not that salvation is complicated, but it must be gained, not through our own merit, of course, but through obedience to Christ and his Gospel as he gave it to his Church.