This is what I understand from this Brother Eli's video:
In Lk. 23:42 where the thief requested that Jesus should remember him in his kingdom, he professed his faith in the Lord Jesus Christ. That faith was enough to save him because he had only a short time to live. But for someone who still have time to live, such profession of faith is not enough.
2Peter 1:5-8 admonishes us to add virtue, knowledge, temperance, patience, godliness, kindness and charity to our faith. After being purged of sin, we need obedience to the Lord.
1Cor. 13:13 says faith, hope and charity work together. And 1Cor. 13:1 emphasizes that love is the greatest.
The thief on the cross had no chance to demonstrate love or good works. This was why his faith was sufficient to bring him salvation.