Mark 2:13 And he went forth again by the sea side; and all the multitude resorted unto him, and he taught them.
Again, teaching the word of God was first. He brought truth and grace.
Mark 2:14 And as he passed by, he saw Levi the son of Alphaeus sitting at the receipt of custom, and said unto him, Follow me. And he arose and followed him.
Levi, a publican and tax collector, was Matthew, who would give his gospel account to us later. He started out coming from the position of a despised tax collector. The collectors asked more than what the tax was, for their own wages. He arose and followed Jesus, as did the fishermen.
1 Cor 1:26 For ye see your calling, brethren, how that not many wise men after the flesh, not many mighty, not many noble, are called:
1 Cor 1:27 But God hath chosen the foolish things of the world to confound the wise; and God hath chosen the weak things of the world to confound the things which are mighty;
1 Cor 1:28 And base things of the world, and things which are despised, hath God chosen, yea, and things which are not, to bring to nought things that are:
1 Cor 1:29 That no flesh should glory in his presence.
Mark 2:15 And it came to pass, that, as Jesus sat at meat in his house, many publicans and sinners sat also together with Jesus and his disciples: for there were many, and they followed him.
More followers from this group of people. We are not to despise a group of people, because of what they are. Jesus will make them new in him, if they respond to the gospel word. He loved them, so should we. Not the sins, but their soul that needs the Lord.
Mark 2:16 And when the scribes and Pharisees saw him eat with publicans and sinners, they said unto his disciples, How is it that he eateth and drinketh with publicans and sinners?
I suppose that was a natural thing to say, for are we not to be separate?
2 Cor 6:17 Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you,
2 Cor 6:18 And will be a Father unto you, and ye shall be my sons and daughters, saith the Lord Almighty.
Separate from sin, but we are to witness to sinners. The Pharisees were not witnessing to Gentiles, they despised them as they despised tax collectors and sinners.
Mark 2:17 When Jesus heard it, he saith unto them, They that are whole have no need of the physician, but they that are sick: I came not to call the righteous, but sinners to repentance.
Jesus knew hearts and he knew who would follow him. We do not know who will be saved, so we witness to everyone who will hear.