Referencing the gospel accounts of Christ's actions and teachings, how does He clearly define a Christian?
Jesus didn't, because the word "Christian" only appeared much later. Acts 11:26c:
The disciples were called Christians first at Antioch.
To the extent that "being Christian" means following Christ in imitation of Him, Christ gives the answer in Matthew 16:24: " Then Jesus said to his disciples: If any man will come after me, let him deny himself, and take up his cross, and follow me."
In commentary on this verse Saints Chrysostom, Euthymius, and Theophylactus, show that free will is confirmed by these words:
"Do not expect, O Peter, that since you have confessed me to be the Son of God, you are immediately to be crowned, as if this were sufficient for salvation, and that the rest of your days may be spent in idleness and pleasure. For, although by my power, as Son of God, I could free you from every danger and trouble, yet this I will not do for your sake, that you may yourself contribute to your glory, and become the more illustrious." (St. Chrysostom, hom. lvi.)
Jesus generally talks about His Kingdom when He talks about the church. But He does have things to say about who belongs among His followers:
Mark 9:33-41 (ESV)
33 And they came to Capernaum. And when he was in the house he asked them, “What were you discussing on the way?” 34 But they kept silent, for on the way they had argued with one another about who was the greatest. 35 And he sat down and called the twelve. And he said to them, “If anyone would be first, he must be last of all and servant of all.” 36 And he took a child and put him in the midst of them, and taking him in his arms, he said to them, 37 “Whoever receives one such child in my name receives me, and whoever receives me, receives not me but him who sent me.”
38 John said to him, “Teacher, we saw someone casting out demons in your name, and we tried to stop him, because he was not following us.” 39 But Jesus said, “Do not stop him, for no one who does a mighty work in my name will be able soon afterward to speak evil of me. 40 For the one who is not against us is for us. 41 For truly, I say to you, whoever gives you a cup of water to drink because you belong to Christ will by no means lose his reward.
He sets the bar of membership much lower than any church I know (including my own):
- No matter who you are or what you've done, if you act in Jesus' name, you are His follower.
But we can't forget the immediate context:
Mark 9:30-32 (ESV)
30 They went on from there and passed through Galilee. And he did not want anyone to know, 31 for he was teaching his disciples, saying to them, “The Son of Man is going to be delivered into the hands of men, and they will kill him. And when he is killed, after three days he will rise.” 32 But they did not understand the saying, and were afraid to ask him.
If we want to follow Him, be imitators of Christ, or Christians (literally "little Christs"), we need to follow Him where ever He goes: even to death. We also believe we can follow Him in Resurrection.